Click here for reference.
Note also that an output can be used only once. That's why there's a concept of "change address" in the bitcoin ecosystem: if an output of 10 BTC is available for me to spend, but I only need to transmit 1 BTC, I'll create a transaction with two outputs, one with 1 BTC that I want to spend, and the other with 9 BTC to a change address, so I can spend this 9 BTC with another private key that I own.
So what will happen if an output of 1 BTC is available for me to spend, but I want to spend 10 BTC?
Note: It is a general question and is not specific to any API feature.