The CA stands for Claim Acknowledgment. The 277CA was formerly known as the unsolicited claim status response. This transaction is used to tell the sender of a claim in detail whether the claim was rejected, if there is additional information necessary and maybe even if the claim is already adjudicated. This transaction is unsolicited, meaning that the trading partner did not send a 276 status request to receive this 277CA. This transaction is usually sent by the receiver after a 837 claim file came in and  has been pre-processed.