For the GNU compiler:
The number of significant initial characters in an identifier (C90 6.1.2, C90, C99 and C11 188.8.131.52, C99 and C11 6.4.2).
For internal names, all characters are significant. For external names, the number of significant characters are defined by the linker; for almost all targets, all characters are significant.
Whether case distinctions are significant in an identifier with external linkage (C90 6.1.2).
This is a property of the linker. C99 and C11 require that case distinctions are always significant in identifiers with external linkage and systems without this property are not supported by GCC.