In addition to labels that follow the rules for M names, M accepts labels consisting only of digits. In a label consisting only of digits, leading zeros are considered significant. For example, labels 1 and 01 are different. Formalists may immediately follow a label. A Formalists consists of one or more names enclosed in parentheses (). Formalists identify local variables that "receive" passed values in M parameter passing. For more information, refer to the section on "Parameter Passing" in this chapter.

In GT.M, a colon (:) delimiter may be appended to the label, which causes the label to be treated as "local." Within the routine in which they appear, they perform exactly as they would without the trailing colon but they are inaccessible to other routines. Using local labels reduces object size and linking overhead, for both ZLINK and host linking.