How Store CSS as files works for custom mods?

[CvP]

I'm not mistaken, I should use this code to include custom css in custom template page

<vb:if condition="$vboptions['storecssasfile']">
{vb:cssfile mycss-rollup.css}
<vb:else />
{vb:cssfile mycss.css}
</vb:if>

where my css template is named mycss.css

I am aware that cssrollups are defined in /includes/cssrollup_xxx.xml
But I'm unable to find any documentation relating to this.
I can place my own rollup xml in there but how do I let vbulletin know that i have a rollup xml in there?

edit: also, can someone explain what is the process vBulletin follows for store css as file setting?