boot.zfs.extraPools

Name or GUID of extra ZFS pools that you wish to import during boot.

Usually this is not necessary. Instead, you should set the mountpoint property of ZFS filesystems to legacy and add the ZFS filesystems to NixOS's fileSystems option, which makes NixOS automatically import the associated pool.

However, in some cases (e.g. if you have many filesystems) it may be preferable to exclusively use ZFS commands to manage filesystems. If so, since NixOS/systemd will not be managing those filesystems, you will need to specify the ZFS pool here so that NixOS automatically imports it on every boot.