What does pkg_add need?

[Aldert Nooitgedagt]

Are there any specific docs that tell me what pkg_add needs?

If a Makefile creates a fully functional port, it may be that pkg_add will
fail.

Is it obligatory to add "@exec mkdir" for every directory that is created in
Makefile?

Is it obligatory to add @owner/@group to plist for every chown in Makefile?
Or should Makefile not contain any chown/chmod and should that always be
handled by pkg-install?

Thanks.