I know this question has been asked before, but I can't seem to find information about it in Apple's documentation; maybe some of you guys did.
A lot of Objective-C code has cross-file constants in a
Apple's recommendation is
Define constants for strings used for such purposes as notification names and dictionary keys. By using string constants, you are ensuring that the compiler verifies the proper value is specified (that is, it performs spell checking).
Admittedly they are inconsistent about this sometimes.