This 'guide' is meant specificly for simfiles that you've created, for the purpose of adding a bit of flare to them.
Other tags in simfile data are used for several very useful things. Though before I even get into these, let it be known, that DWI is heavily limited in its capabilities in comparison to SM. Such as not being able to produce mines, rolls, having a limit to maximum backgrounds changes and things of that nature. As a rule of thumb, since you are using StepMania, it would be wise to stick to StepMania (SM) form. That having been said, I won't be discussing DWI form here, so let's do this.
Keep in mind that these are ordered by function and not by the tag the functions use, that way I can address tags in both the DWI and SM forms simultaneously.
Titles and Artist
#TITLE:;
#SUBTITLE:;
#ARTIST:;
Extremely simple here, simply fill these tags in appropriately with the song's details. Example pulled out of a hat...
#TITLE:Sidology Episode 2;
#SUBTITLE:(Trinity);
#ARTIST:Machinae Supremacy;
Banners
#BANNER:;
These can be named whatever you want, as long as the computer will allow, simply enter the image's filename in the tag. Example...
#BANNER:My Banner.png;
Backgrounds
#BACKGROUND:;
Same story as Banners. Example...
#BACKGROUND:That backdrop.png;
Background Changes
#BGCHANGES:;
This is where background changes are made, generally used for videos, sometimes for elaborate slideshows. Using this by hand can be extremely complicated. I suggest that if you're adding background changes to use the editor in StepMania 3.9 final. However, if you must do this by hand, for one reason or another. Here is the breakdown...
This is the most simple way I can explain this... the tag will be in this form - MEASURE=FILENAME=RATE=FADELAST=REWINDMOVIE=LOOP (comma) repeat. MEASURE is self explanatory, representing the measure within the song on which the change will occur. FILENAME is simply where you enter the name of the file that the background will change to, this can be an image or a video file. RATE appears to be how fast the change will occur and will be entered in a ratio of 100% = 1.000. FADELAST, REWINDMOVIE, and LOOP are logic entries, meaning that 0 = no and 1 = yes. FADELAST determines if the the change coming in will fade in. REWINDMOVIE appears to mean that if the same video file is used twice, then whether or not it is starting from the beginning the second time. And LOOP is whether or not a video will loop when it ends. Remember that not all of this is completely bug free on StepMania and the bugs that you experience will depend entirely on the build of StepMania that you are using.
In this example, on measure 6.000 the background changes to an image with no extra effects, then on measure 24.000 it switches to a video at 90% with all of the effects...
#BGCHANGES:6.000=Image.png=1.000=0=0=0,24.000=Video.avi=0.900=1=1=1;
Translation
#TITLETRANSLIT:;
#SUBTITLETRANSLIT:;
#ARTISTTRANSLIT:;
These tags are meant for the use of substituting the Title, Subtitle, and Artist names with an image. It is meant to be used for the purpose of replacing a Romanji (Japanese written in roman lettering, such as this) title of a song with it's proper Japanese (or other language, who knows?) lettering. It will not always be used, in fact, I'd say that it's probably quite rare. The tag is properly filled in with the name of the proper image file. Examples...
#TITLETRANSLIT:japaneseletters.png;
#SUBTITLETRANSLIT:evenmorejapaneseletters.png;
#ARTISTTRANSLIT:andevenmorejapaneseletters.png;
Lyrics
#LYRICSPATH:;
When filled in, this tag points to a file that causes song lyrics, or even commentary at the bottom of the screen while the song is being played. It is not very commonly used but not entirely ignored either. Example...
#LYRICPATH:words.lrc;
CdTitles
#CDTITLE:;
When an image file's name is placed in this tag, it appears as an icon on the top right of the banner of the song during song selection (depends entirely on the theme being used). The image file being used for this should have fairly small dimensions. Example...
#CDTITLE:myicon.png;
Music
#MUSIC:;
This tag is meant to point to the file used for the music given for the simfile chart. Unless the process is completely botched this tag should fill in all by itself. However, in the extreme instance that it doesn't, or if the format of the audio is changed, it may become necessary to fill it in or alter it. Example...
#MUSIC:The Song.mp3;
BPM/Tempo and Changes
#BPMS:;
BPM is one factor of any given simfile that must be absolutely perfect or close enough that a human being cannot tell the difference. This tag is meant to be filled with numbers. The numbers it is filled with determines the scroll rate of the measures on which you place steps. If this number is off then the steps will either scroll too slow, falling behind the song, or too fast, getting ahead of it. The first number in the tag WILL be 0.000, with no exceptions, that number represent the measure on which the BPMs take affect, measures are represented with different numbers, it is wise to note them when trying to do things of this nature within the StepMania editor. In this example, the bpm begins at 126.900 then changes to 95.175 on the 65th measure...
#BPMS:0.000=126.900,65.000=95.175;
Stop Time
#STOPS:;
When this tag is used, the steps hold perfectly still on the measure designated for the amount of time entered. It is filled in in the same format as #BPMS:; but the second value this time, instead of a BPM, it is a time value. In this example, we have a stop at measure 11 for exactly 0.481 seconds, then another at measure 82...
#STOPS:11.000=0.481,82.000=0.481;
Offset or Music Gap
#OFFSET:;
Obviously, nearly no song will ever fall into line perfectly with the measures on StepMania. Thus offset is used to tilt the play time of the music by milliseconds in order to line up with the available measures. It would be wise to note that if your BPM is wrong, tweaking this will do little or no good at all. It can be entered with a positive or negative number, but it is usually best to edit this within StepMania or your chosen editor program. Example...
#OFFSET:-0.404;
Song Preview during Song Selection
#SAMPLESTART:;
#SAMPLELENGTH:;
These tags are for the customization of the song preview heard during a song selection. When you highlight a song on the music wheel (or whatever it may be in your theme) you hear a small piece of the song you're on in order to give you an idea or feeling for what the rest of the song may be. It's generally a good idea to make certain that you choose a portion of the song that would encourage people to play the song. The first tag should be entered with a time value, exactly when in the song should the preview begin, while the second tag should be entered with a time value determining how long the preview will last. Example...
#SAMPLESTART:38.293;
#SAMPLELENGTH:14.083;
Selectability
#SELECTABLE:;
The purpose of this tag is to tell StepMania whether this simfile can be selected from the song wheel. For the life of me, I cannot think of any good reasons to do this, but you have the option. Example...
#SELECTABLE:YES;
or #SELECTABLE:NO;
Q: Tags pointing to images, what file types work?
The accepted image filetypes are: BMP, GIF, JPG, and PNG. Tags that involve images include: #BANNER, #BACKGROUND, #TITLETRANSLIT, #SUBTITLETRANSLIT, #ARTISTTRANSLIT, #BGCHANGES, and #CDTITLE.
Q: Other tags?
#CREDIT:; - Meant for giving credit, say, to a simwriter or organization.
#KEYSOUNDS:; - meant for IIDX style key sounded notes, proper use is unknown.
#MUSICBYTES:; - garbage, throw it away.
Q: Videos/Lyrics? (Anything partaining to them)
Different FAQs, not up yet.
Q: Accepted music filetypes?
WAV, MP3, and OGG. In order of efficiency.
Feel free to send any further questions.