man psで調べてみた
a Lift the BSD-style "only yourself" restriction, which is imposed upon the set
of all processes when some BSD-style (without "-") options are used or when
the ps personality setting is BSD-like. The set of processes selected in
this manner is in addition to the set of processes selected by other means.
An alternate description is that this option causes ps to list all processes
with a terminal (tty), or to list all processes when used together with the x
option.
-A Select all processes. Identical to -e.
-a Select all processes except both session leaders (see getsid(2)) and
processes not associated with a terminal.