Bets are placed at price that is set by one of "Price setting" rules. In most cases you will be using "Price settings->Price rules" rule to set price at which bets will be placed by your strategy.
Default option is to use currently available price. This is best available Back price for Back bets and best available Lay price for Lay bets.
Sometimes price changes while bet is being sent to server and this can cause your newly placed bet to be unmatched. To increase chances of your bet getting matched instantly you can set "Price rules" rule to ask for worse price by X ticks or X percents. This means that this rule will take currently available price and adjust it by X ticks (or by set percentage) and then place your bet at that worse price.
Here are few examples:
a) Back price is 2.0 and you set to ask for worse price by 4 ticks. Bet will be placed at price 1.96 as that is 4 ticks lower than price 2.0 by Betfair odds ladder.
b) Lay price is 2.0 and you set to ask for worse price by 5 ticks. Bet will be placed at price 2.08 as that is 4 ticks higher than price 2.0 by Betfair odds ladder.
Betfair always matches your bet at best available price up/down to price that you requested.
This means that your Back bet would first be matched at price 2.0, then the remaining part would be matched at 1.99, then 1.98, then 1.97 and finally at 1.96. If there is still unmatched part then that would stay unmatched at 1.96.
Your Lay bet would first be matched at price 2.0, then the remaining part would be matched at 2.02, then 2.04, then 2.06 and finally at 2.08. If there is still unmatched part then that would stay unmatched at 2.08.
You can also set bets persistence that will tell Betfair what to do with unmatched bets at in play. To set bets persistence you can use "Staking->Bets persistence" rule and set it to keep unmatched bets alive when market goes in play or to match your bets at Betfair Start Price. For more details on this you should see bets persistence article.
