s3xmodit is format primarily used for '[[OHB]s which allows for the submission of any of these tracker modules:

.s3m
.xm
.mod
.it

Thus, "s3xmodit!"

This category grants Mixist points.

'[#[Software]
.s3m, .xm, and .it were originally made in DOS trackers. s3m was made in Scream Tracker 3, .xm was made in Fast Tracker II, and .it was made in Impulse Tracker. 

Scream Tracker 3: http://www.pouet.net/prod.php?which=13351
Fast Tracker II: http://www.pouet.net/prod.php?which=13350
Impulse Tracker: http://www.pouet.net/prod.php?which=13366

If you want to use any of these programs you will need a DOS emulator. However, there are many alternatives to these programs that run on modern operating systems.

Milkytracker (.xm and .mod only): http://www.milkytracker.org/
OpenMPT (.xm, .mod, .it, and .s3m): http://openmpt.org/
Schism Tracker (.it and .s3m): http://schismtracker.org/

Effect commands:
xm: http://battleofthebits.com/lyceum/View/FastTrackerII%20Effects%20Commands
it: http://battleofthebits.com/lyceum/View/ImpulseTracker%20Effects%20Commands

If you want details on the .mod format (which came from the Amiga) go here: http://bit.ly/193CeuX

'[#[In Majors]
In a major compo with a s3xmodit category, you are normally allowed to just submit any s3xmodit file with no restrictions (except under 10 mb) that you desire.

See:
http://battleofthebits.com/arena/Battle/1035/Spring+Tracks/
http://battleofthebits.com/arena/Battle/1403/Spring+Tracks+II/

'[#[In OHCs]
A sample pack is provided and normally you are only allowed to use these samples, you may not add any samples to the sample pack (unless explicitly stated that you may add samples).

Sometimes text rules are given too with a theme instead of a sample pack (though these are less common). Also, both a sample pack and text rules may be given in a zip file.

'[#[Size-based Formats]
For large battles, module categories are usually grouped by size (8k, 32k, ect...) but will still accept any of the above mentioned modules.  For size based formats, categories less than 64k will grant Chipist points.

'[[mod8k (format)[mod8k]
'[[mod12k (format)[mod12k]
'[[mod16k (format)[mod16k]
'[[mod24k (format)[mod24k]
'[[mod32k (format)[mod32k]
'[[mod48k (format)[mod48k]
'[[mod64k (format)[mod64k]