Content Types are great, but can cause a little confusion. Because you normally define a content type at a site level, that’s pretty much how we think of them – as centrally defined types of item. Often, we actually create these content types on the root site of a site collection, because all subsites will be able to use them then.
However, this isn’t really the case. We do have site content types – but we also have list content types. These are the content types that are actually used on the lists themselves, and they are children of those site content types. This can be most easily seen by clicking on a content type on the List Settings page.
Notice that our ‘Example Travel Expenses’ content type says it has a parent of… …’Example Travel Expenses’! This is our List Content Type telling us that it’s parent is the Site Content Type of the same name. Click on it, and it’ll take you to the Site content type description, and you can work on up the chain of content types until you reach Item.
A consequence of this is that, as our content types are actually used by lists, I can’t think of a way to use a Site Content Type directly (though I may be wrong about that).
That are also issues related to this in terms of modifying content types, but that’s the subject for another post…