425nesp's questions

I was reading the manpage for systemctl reload and found this sentence about reload.

reload PATTERN...
    Asks all units listed on the command line to reload their
    configuration.

http://man7.org/linux/man-pages/man1/systemctl.1.html

How does systemd do that? What API does systemd use to ask a unit to reload its configuration?

I also found reload-or-restart.

 reload-or-restart PATTERN...
     Reload one or more units if they support it. If not, restart them
     instead.

How does systemd know if the unit supports reload?

Let's say I write a mine.service file. Then I use systemctl enable mine.service.

If I later decide to edit mine.service, do I have to tell systemd that mine.service was changed? If so, how do I do that?