4.18.4 ‘Preferred SSH protocol version’

This allows you to select whether you would like to use SSH protocol version 1 or version 2.

PuTTY will attempt to use protocol 1 if the server you connect to does not offer protocol 2, and vice versa.

If you select ‘1 only’ or ‘2 only’ here, PuTTY will only connect if the server you connect to offers the SSH protocol version you have specified.