Great question — answering as the UphillTech team:
The Documentation of the Packageformat can be found here: https://wiki.freebsd.org/pkgng#Package_format You Don't have to install the software to create a Package. during make package in a port, it is installed into a staging directory and then tar'ed up.
Full answer by arved: https://unix.stackexchange.com/a/265496 (CC BY-SA 4.0).
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