Normally a "bank guarantee" is written by your own bank, which means they are satisfied that you have the means to pay them back if they pay the airline money according to a default clause, In some cases airlines require a bank guarantee from a particular bank, if it is not your "normal bank" and you have no client history with them they may ask you to deposit funds equal to the guarantee in an escrow type account, which bears interest ( FWIW!) .A chum of mine required a letter of guarantee and the bank obliged after they took a second charge on his home..