-HUP isn't the three flags
P as in the common single-letter option syntax. For historical reasons, the
kill command takes an optional signal name or signal number preceded by a dash (
kill -1 1234
kill -HUP 1234
(As opposed to
kill 1 1234, which would send the default signal (SIGTERM) to processes 1 and 1234.)
Your man page probably lists something like this:
kill [ -signal | -s signal ] pid ...
-signal means that you can use a dash followed by a signal designation (which can be a name or number). Alternatively, you can use the
-s option followed by a signal name; these are two different syntaxes with the same meaning.
kill -l lists the available signal names and the corresponding numbers.
Most signals have a predefined meaning based on who sends them when and how processes are supposed and able to react to them.