There are 3 Staking modes 2 of which affect how the software calculates the stake to be used when placing a lay bet.

The default staking mode when you first run Betting Assistant is 'Stake' meaning that when you place a lay bet the amount used will be the actual stake that you requested.

However you can click on this in the info bar and it will change to staking mode: liability and click again and it will change to staking mode: Payout. 

Click once more to revert back to staking mode: stake
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In liability mode when you place a lay bet the software adjusts the stake so that the liability you end up with is the amount you originally requested.

And in payout mode the stake is adjusted so that the liability is the amount that the backer will win if the selection wins i.e. the backers original stake plus your liability equals the amount you originally requested.