SMXML Control Format | Informal and Unofficial Specification
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There is no formal DTD for SMXML as of this writing, so we can only go on the
example file and comments written about the format from developers.

Chris Danford writes[1]:
"I've been finishing a new in-game download system. The goal is to:
- Have as few user clicks as possible to install a package (or many packages)
- Have one cross-platform method of installing packages

The new file association - .smxml - contains control data that StepMania will use
on launch. The file format currently supports one or more packages to install."

The canonical sample SMXML file[2] looks like this:
<SMXML>
	<Credentials>
		<Username>chrisdanford</Username>
		<PasswordToken>#&DJASDJ#$&$JSD</PasswordToken>
	</Credentials>
	<Install>
		<URL>http://stepmaniam3.com/sm/data/files/Songs_DJMcFox_PlagueMixOne.smzip</URL>
	</Install>
	<Install>
		<URL>http://stepmaniam3.com/sm/data/files/Songs_DJMcFox_PlagueMixTwo.smzip</URL>
	</Install>
</SMXML>

The Credentials section seems to be optional, but is useful to know about:
"In the future, it might also be used to pass login credentials or other tokens
from the web site to StepMania for things like in-game uploads (instead of going
to stepmaniashare.com)"[1]
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[1] http://www.stepmania.com/forums/showthread.php?t=24203
[2] http://www.stepmania.com/test.smxml