Robert M, yes it is open source.
Ed, if you share a file then the recipient can have either version (the standalone or the installable).
To share a file you need premium.
To receive a file you can be a free or premium user.
Exporting keys is something totally different; it’s for backing up your key-pair. AxCrypt 2 does this automatically (by keeping a copy on their servers) so you don’t have to. The option is included just in case you wish to keep your own copy. If you don’t understand what the option does, don’t worry, you won’t need to use the feature.