External and internal Identifier

Question

I know to code in C well but I thought of learning C from the book C - The Complete Reference by Herbert Schildt. Here is a quote from Chapter 2:

In C89, at least the first 6 characters of an external identifier and at least the first 31 characters of an internal identifier will be significant. C99 has increased these values. In C99, an external identifier has at least 31 significant characters, and an internal identifier has at least 63 significant characters.

Can somebody explain what does it mean to be significant?

Answer 1

That means that it is used within the compiler to differ between different names.

E.g. if only the first 6 characters are significant, when having two variables:

int abcdef_1;
int abcdef_2;

They will be treated as the same variable, and possibly the compiler will generate a warning or error.

About the minimal significance:

Maybe the compiler/assembler can handle more, but the linker cannot. Or maybe external tools which are out of control of the manufacturer of the assembler/linker can handle less, thus a minimum value (per type, internal/external) is defined in the C standard(s).