On the Land Units page, you have the option to combine land units.
This is useful to be able to join different land units that are not connected to be treated as one land unit.
To combine land units, you can tick the box in the land units properties, which will then display a dropdown of other land units to join the currently open land unit to.
You can also combine land units in bulk by ticking ‘Combine land units’ on the top of the map, selecting land units and then double clicking inside one of the selected land units to bring up the option of which land units you want to join ALL selected land units to.