flag

See affinity tag.

We tend to avoid this term because it means so many things. It may mean a command-line "switch" that takes no argument itself (such as Perl's -n and -p flags) or, less frequently, a single-bit indicator (such as the O_CREAT and O_EXCL flags used in sysopen).

In data transmission or processing, an indicator, such as a signal, symbol, character, or digit, used for identification. Note: A flag may be a byte, word, mark, group mark, or letter that signals the occurrence of some condition or event, such as the end of a word, block, or message. Synonym field tag.