In GNUPG Kleopatra, what does “+RSA” do when generating a key?

A customer of ours wants us to create PGP keys similar to those created by GPG’s Kleopatra utility when the GUI option “+RSA” is checked. We don’t use Kleopatra, and would like to know just what the “+RSA” option does so that we can use the equivalent functionality in the PGP software that we do use.
I have downloaded and installed Kleopatra, read the documentation, and run it with verbose debug logging, but unfortunately I am none the wiser for having done so.

Does anyone know what +RSA does?

Thanks.