There are no reserved words in the operating system assembler language. The mnemonics for instructions and pseudo-ops are not reserved. They can be used in the same way as any other symbols.
There may be restrictions on the names of symbols that are passed to programs written in other languages.
The atof subroutine.
.comm Pseudo-op, .csect Pseudo-op, .double Pseudo-op, .dsect Pseudo-op, .float Pseudo-op, .lcomm Pseudo-op, .tc Pseudo-op, .toc Pseudo-op, .tocof Pseudo-op.