Integer constants fall into groups, where all constants in each group must have unique values. In cases where integer constants are intended to be combined using bitwise logical expressions, there are additional requirements, specified elsewhere (e.g. Section Opening a File). A different constraint exists for constants like MPI_ANY_SOURCE, which must be negative, because any non-negative integer may be a valid rank.
The MPI ABI reserves all unused values up to 16384 for integer constants, to allow for adding new constants without creating a noncontiguous range. For example, implementations may define their own extensions for MPI_COMM_SPLIT_TYPE that use non-standard values of the split_type argument -- these integer constants must exceed 16384.