When sending funds from Coinbase to an off-site address, you may be asked "are you sending to another digital currency service or exchange?" If you are not, you can simply click "No." If you are, you must click "Yes" and specify the URL of the website of the destination digital currency service or exchange.
When sending money to external accounts we are required to gather a bit of information before processing the transaction. Coinbase is federally registered as a Money Service Business (MSB). In order to comply with federal regulations, we must know if money is sent from a Coinbase account to a financial institution, in this case a digital currency service or exchange.