Combination values are bit-encoded constants that can or must be added to other constants. For example, if you want to allocate an image buffer to hold compressed data, you would specify M_IMAGE and add the value M_COMPRESS:
The preamble to a combination values table describes whether or not the combination values are optional or essential. Note that essential combination values cannot be used independently. Generally, combination values are listed in a separate table, below the table in which the value is to be combined.