[systemd-devel] a little help with $MAINPID please

[lux-integ]

Greetings

I am an absolute beginner with systemd.  I came across the gentoo   systemd 
script 
( http://en.gentoo-wiki.com/wiki/Systemd )
for mysql-daemon.  It has these lines:-

ExecStart=/usr/sbin/mysqld --pid-file=/var/run/mysqld/mysqld.pid
ExecStop=/bin/kill -15 $MAINPID
PIDFile=/var/run/mysqld/mysqld.pid

I am curious and  puzzled about two things.
a) What  is $MAINPID  which I have seen in so many systemd scripts
i.e. where is is set? or how is it determined?
b) what are the advantages or disadvantaes of the following line 
ExecStop=/bin/kill -15 $(/bin/pidof mysqld)
over
ExecStop=/bin/kill -15 $MAINPID


thanks in anticipation
regards
lluxInteg