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…

This is sort of relevant to an earlier post on the Document Information Panel, and showing fields in it.

The behaviour depends on if you’ve enabled ‘Allow management of content types’ on the Document Library Settings > Advanced Settings page.

If you’ve not allowed management of content types, well, you just add the column and it’ll appear in the document information panel. That’s great! The new column will not show as belonging to any content types, as the ‘Test‘ column is in the screenshot below:

However, if you have allowed management of content types for the library, things get a little more complicated. When you add a new column, in the ‘Additional Column Settings’, there will be an option for ‘Add to all content types’. If you check this, well, it’ll add that column to all the content types currently on that list. This will make it appear on the document information panel. This is what I did with the ‘Test2′ column above (but before I added the Picture content type to the library).

(As a side note, the content types on the list are actually ‘children’ of your Site content type rather than instances of it. This means that if you update the content type on that list, it won’t update the parent content type, or other lists that use that content type. Similarly, it mean that if you update the parent content type – say, you edit the site content type – you need to update child content types with those changes to affect lists that are using them already. But that really should be another post, sometime.)

If you don’t check that ‘Add to all content types’ option, well, it doesn’t add it to the document information panel. Finally, what if you have added a column all content types, and you add a new content type to the list? Well, your new content type will not have that new column applied to it – and the only way I can see of applying it to the new content type is actually to delete and recreate it. Of course, that means deleting a column that contains data, so that is less than ideal. This is what happened with the Picture content type – I added it afterwards, and you can see that it doesn’t use the ‘Test2‘ content type.

For that reason, be very careful when adding columns to library that is allowing management of content types. If possible, keep the columns in the Site Content Type (i.e. the parent).