3.3.4.3 How Long Can I Make My Variable and Macro Names?

The C Standard indicates that only a number of initial characters in an identifier are significant, but it does not actually state what this number is and it varies from compiler to compiler. For MPLAB XC32, no limit is imposed, but for CCI there is a limit (see 25.3.5 The Number of Significant Initial Characters in an Identifier). CCI Compliant names are more portable across Microchip architectures.

If two identifiers only differ in the non-significant part of the name, they are considered to represent the same object, which will almost certainly lead to code failure.