When you send funds/bitcoin they are sent from the address or addresses on which the coins reside.

Example:

Address A has a balance of 1 bitcoin
Address B has a balance of 0.5 bitcoin
Address C has a zero balance

If you want to send say 1.2 btc then both address A and B will be used. Address C will not be used as there is no funds on that address. So the 'send' address will depend on which address or addresses have funds on them at the time as well as on the amount you are sending.

A sending address can be used more than once as it will be used until the balance is zero. If you send funds to that same address again then it will be used again as a 'send' address.