The character : is permitted in identifiers. There are two exceptions to it being treated as any other symbol character: if a symbol begins with :, it means that the symbol is in the global namespace and that the current prefix should not be prepended to that symbol (see MMIX-prefix). The : is then not considered part of the symbol. For a symbol in the label position (first on a line), a : at the end of a symbol is silently stripped off. A label is permitted, but not required, to be followed by a :, as with many other assembly formats.
The character @ in an expression, is a synonym for ., the current location.
In addition to the common forward and backward local symbol formats (see Symbol Names), they can be specified with upper-case B and F, as in 8B and 9F. A local label defined for the current position is written with a H appended to the number:
3H LDB $0,$1,2
This and traditional local-label formats cannot be mixed: a label must be defined and referred to using the same format.
There's a minor caveat: just as for the ordinary local symbols, the local symbols are translated into ordinary symbols using control characters are to hide the ordinal number of the symbol. Unfortunately, these symbols are not translated back in error messages. Thus you may see confusing error messages when local symbols are used. Control characters \003 (control-C) and \004 (control-D) are used for the MMIX-specific local-symbol syntax.
The symbol Main is handled specially; it is always global.
By defining the symbols __.MMIX.start..text and __.MMIX.start..data, the address of respectively the .text and .data segments of the final program can be defined, though when linking more than one object file, the code or data in the object file containing the symbol is not guaranteed to be start at that position; just the final executable. See MMIX-loc.