From Facebook Developer Wiki

In different contexts, you are allowed to do different things in FBML. These contexts are formalized with a concept called "flavors." A concrete example of this is a Facebook Canvas page vs. a profile box. On a canvas page, you can have fb:iframe tags, while you can't do this in a profile box. If you write some FBML that is disallowed by the flavor that is being used to interpret it, that particular tag and its contents will not be rendered at all, and an error message will be recorded.

There are four types of flavors: All, Canvas, Profile, and Publisher.

See also Allowed FBML and HTML Tags.