Liability
This is the amount you could lose in your worst-case scenario. When backing it is simply the stake which you place. When laying however it is the amount it will cost you if your selection wins. Either way, it is how much you are risking.
Ensure you monitor how much you are liable for.
NB - Be very watchful of where you put the decimal point. If you want to lay a bet at 4.2 make sure it is just that and not 42... otherwise it will be an expensive mistake should your selection win. One thing's certain though, Betfair will only let you risk as much as is in your account and at the best available odds.
If you lay multiple selections in a field your liabilities don't accumulate, your bets offset each other providing the market only has one winner. Use the 'What if' function (as detailed in the next article) to test it out for yourself.