/bin/rmwill always refer to a binary command rmon your system. If you just write rm abc.txt, this could happen:
Your shell implements rmdirectly as a built-in function, or there is a shell function called rm(an external command does not start).
rmwas previously a pseudonym (s alias rm=<substituted-command>) to mean something else. The aliased command is usually similar to a function, but this is optional.
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