Finding out property list (plist) attributes and/or processes
Each application can choose what to store in their plist files. The names of the keys and the different possible values are determined by the application, which can be developed by Apple or by any third party, and there is no standard.
To find different combination of keys/values usable in a plist file, you can do the following:

Read the documentation. But often Apple doesn't provide full documentation.

Find existing plist file and modifiy them, by deducing and trying different keys/values.

Run the command strings on the executable to dump out all text. (Thanks to mmmmmm for this).

Look at the source code if the application is open source, reverse engineer it otherwise.