CTS informs that the module is busy when it is high. It can be busy if it receives a packet over air or if it transmit a packet. Note that it is busy and CTS is high both during the actual reception/transmission and during baseband handling of the data to/from UART. I have enclosed a time plott when the slave send 5 bytes over UART and the master receive the Header + 5 bytes. CTS is enabled and informs when the module is busy.
Last Update: November 9, 2017