[Pierre]

The case is the unit that holds the gears in. It has an input flange that connects to the driveshaft and 2 output flanges that connect to the axles and the wheels.
The cover bolts up tp the back of the diff case. The same diff may have different covers depending on the car it goes into.
The 318ti is an E36 but it uses a rear suspension setup from the E30. (The Z3 convertible and coupes do too.) Standard E36 parts do not fit in the back of the ti. E30 parts too.
The E30 came with 2 styles of diffs. The small case installed on 4-cylinder cars and the big case installed in 6-cylinder cars. The e28 also used the same differential case. There were many ratios offered on the many variations of these cars. Small and big cases are interchangeable.
The ti has the same case as the small version of the E30, since it's also a 4-cylinder. But you can install a big case diff on the ti. You will then need the cover that fits the big case E30s.
I hope this helps.
Go to real OEM and look at diffs, gear ratios and interchangeability of parts.