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).