I think that hole, as SWEET states, is for filling the u-joint enclosure, but lubricant also moves between the transmission and that enclosure. By adding lubricant via that hole (after opening/closing the u-joint enclosure), you ensure that the fluid level in the whole assembly (transmission and u-joint enclosure) does get too low (from fluid moving from the transmission to the u-joint enclosure - seeking "a level").
If I am wrong, we'll hear about it.
