Rear Seal stops oil from escaping from the rear of the crankshaft as the engine runs. Rear Seal is hidden by the transmission as this is where the transmission connects to the engine. If Rear Seal is bad, oil will come out between the Transmission and Engine.

Depending on whether the car is front wheel drive or rear wheel can affect the complexity of the fix. In both cases the transmission will need to be moved away from the engine to gain access to the seal. This will involve removing Transmission mounts and maybe engine mounts. It may also require the removal of Transmission or even the Engine depending on how the car is designed.

Rear Seal Leak will damage clutch.  Manual (Stick Shift) cars have a clutch beside the Rear Seal. When Oil gets on the clutch, it causes the transmission to slip and if the leak is left long enough then the clutch will fail. The clutch may need to be replaced at the same time the seal is replaced. When driving with an oil damaged clutch, the car may judder as oil soaked bits of clutch rub against the clutch plate causing slippage at random moments.

Additive For Oil Leaks are chemicals added to your engine oil that try to stop leaks. They do this by filling in small holes in Oil Seals and by causing Oil Seals to expand/soften. Only problem with them is sometimes they make the Oil Seal expand too much which cause the Oil Seal to destroy itself. This will make a small oil leak into a big oil leak.

Crankcase Breather allows pressure which may build up in the oil pan to escape. It might be possible that the Crankcase Breather is blocked causing pressure build up in the crankcase. If this happened then oil might get pushed out the rear seal by the pressure.