xmlns="http://messenger.msn.com/messengercontent/1.0" and version="1.0" are constant throughout all the files I've looked at, so don't change these. type will obviously change with what kind of content you are going to be making, but here on the first line it is always theme, even if you intend to only put one content type in your file. I'm reasonably certain partnerid doesn't do much of anything - to wit, the MCO containing the signature sounds included with WLM 2009 still has "TEST" in this field and some MCOs I have on file don't include this field at all - but don't go too crazy with it anyways.       id is seemingly an optional field - here, the count always starts at one.   Here, type must be changed for the content you're defining on the current line - this is a list of all the types available to you:   background usertile wink dynamicpicture emoticon dynamicbackground sound scene   mimetype generally must match the file format your content is in; the odd ones out are the dynamic content requiring Flash, which live in nested CAB files and for the purposes of Messenger are assigned application/messengercontent.   file is the name of the file. (If you fumble this and specify a file that isn't in the MCO, Messenger will immediately halt processing the whole package upon hitting it; any content successfully parsed and installed prior to that point will remain installed.)   stamp is not needed for non-dynamic content, and seeing as we can't create any dynamic content ourselves at present don't worry about it.   I'm not sure what contenttype is supposed to do - I've seen D, M, and P in use, but what difference it makes is unclear.   contentid probably won't make much of an impact on much of anything, but please try your best to come up with a unique string of characters for each item unlikely to be used by anyone else in case a duplicate ID really does cause conflicts.   [type]:default (always preceded by the content type defined previously) sets whether or not the content on this line should be set as the user's new default upon installation. I don't know what happens if you set this to true for more than one item for the given content type, and I don't feel like finding out so don't make that mistake. \[type]:displayname (again always preceded by the content type defined previously) defines the user-facing name of the content in the appropriate menus. [type]:version and xmlns:[type]="http://messenger.msn.com/[type]/1.0" appear to simply be more XML housekeeping.   I have literally no idea what emoticon:namespaceprefix="" is supposed to do.   emoticon:keysequence defines the sequence of characters the user can use to quickly input a custom emoticon; if what you put in this field should happen to already be in use, Messenger will place a number in front of it that increments with each duplicate.