This defaults to version 3, which is fine for most uses.
You might need to select PPK version 2 if you need your private key file to be loadable in older versions of PuTTY (0.74 and older), or in other tools which do not yet support the version 3 format (which was introduced in 2021).
The version 2 format is less resistant to brute-force decryption, and doesn't support any of the following options to control that.