Specifies the initial password for the user, i.e. the password assigned if the user does not already exist. If users.mutableUsers is true, the password can be changed subsequently using the passwd command. Otherwise, it's equivalent to setting the password option. The same caveat applies: the password specified here is world-readable in the Nix store, so it should only be used for guest accounts or passwords that will be changed promptly.
Note that the password option will override this option if both are set.