Whenever you visit a restaurant, online or otherwise, a 20% pre-authorization hold is placed on your debit card in order to ensure that there are enough funds to cover the transaction including tax and tip. Once the transaction settles, it will adjust to the correct amount, however you must have enough in your balance to cover the entire transaction.
Whenever you visit a gas station, a pre-authorization hold is placed on your debit card in order to ensure that there are enough funds to cover the transaction. The amount can range from $20 to $125, depending on the station itself. Once the transaction settles, it will adjust to the correct amount.
Pre-authorization holds can also be placed on your debit card for travel related (i.e. car rentals, hotel stays, etc.) and entertainment related transactions.