UF5 is a short, ground station request for the airframe identity. The DF5 reply is the airframe's identification and is compared to the mode A 4096 code for validity. Also encoded in the DF5 reply is the flight status (FS) field, downlink request (DR) field, utility message (UM) and the identification (ID) field. The FS field is a three-bit word reflecting eight different conditions of airborne, alert and special position indicator (SPI) status of the airframe. The DR field is a five-bit word that will contain the request to downlink certain airframe information. The UM field is a six-bit word that contains transponder communication status information. The ID field is a 24-bit word that contains the mode A identification of the airframe.

UF21 is the long from of a UF5. Where the DF5 is 56 bits long, the DF21 is 112 bits long. The DF21 reply is also known as comm A identity request. The DF21 reply also contains a 56-bit message field for transferring downlinked aircraft parameters (DAPS). To properly test the DF21 reply, the UF21 must contain a reply request (RR) of 17, a designator identifier (DI) of 7 and a reply request subfield (RRS) of 0. The working component of the DF21 reply is the comm B message field or MB. Contained in the MB field is aircraft address (AA), downlink request (DR), flight status (FS) and identification code (ID). These parameters are compared to the DF11 reply for validity. A transponder that does not have an active subsystem that will accept comm A data will not reply to a UF21 interrogation.