Information label combination constraints are used by the labeling software to ensure that no invalid combinations of words are allowed to be specified in a single information label. However, any two valid information labels can be combined by the system by bitwise or-ing the compartment and marking bits. Thus if a combination constraint is specified that (using examples from Appendix B, Annotated Sample Encodings) subcompartment SA and subcompartment SB cannot be combined, an inconsistent situation has arisen. The inconsistency is that SA and SB cannot be combined by entering them in a single information label, yet two separate information labels, each with one of the subcompartments, can be combined to produce a new information label with both subcompartments. Therefore, to avoid such inconsistencies, you should never specify any combination constraints that are not automatically enforced on combinations by the encodings.
Examples of constraints automatically enforced on combinations by the encodings abound when considering inverse words. If two inverse words IW1 and IW2 are constrained not to be combined with the combination constraint:
IW1 ! IW2
then you can be assured that IW1 and IW2 can never be put together as a result of the combination of two labels. Why? Because inverse words combine by having only those inverse words in both of the labels being combined appear in the resulting label. Therefore, if both IW1 and IW2 cannot appear in any single information label, then no combination of information labels can combine IW1 and IW2 together.