Exposure means the same as liability - Each means the maximum amount you could lose if your bet, be it a back or lay bet, be unsuccessful. If you had bet €50 on Sunderland to win, your exposure would be the €50 stake.
If you had laid Sunderland at 1.50 [1/2] not to win for €50, the exposure here would be the same as what you would win had you bet them to win - eg €25
Again, check our betting exchanges basics for a full explanation on liability and exposure.