The Universal Connector was a row of contacts on the bottom surface of the PDA, which mated with springy contacts on the cradle or cable. The "multi-connector," if I remember right, is similar to the connector used on the Nokia 6610 cellphone; the contacts on the PDA are recessed in a slot, and the cradle/cable has a part that goes into that slot.
The Universal Connector and the multi-connector are incompatible.