Why are the Targets memberships checkboxes disabled for all files?

Why are the Targets memberships checkboxes disabled for a开发者_Python百科ll files in my xCode project ?

PS. I'm using xCode 3

---

Target membership defines whether or not certain file is compiled and linked or copied to the bundle for the given target. As there's usually no specific actions for header separately - they're included in source files and those source files are compiled with contents of headers - so there's no sense in target membership for header files.

If you check properties for implementation files then target membership should be enabled