Instruction Responses COMMS
The instruction to "squawk" means to set a code on your SSRSSR —Secondary Surveillance Radar so that you can be identified on radar. If you asked to "squawk with Charlie" this means you also activate Mode C, the altitude readout. If you asked to "squawk with ident" then after you have put on the code you press the ident button which will cause your "icon" to flash on the air traffic controller's display. You should not do either of the last two things (Mode C or Ident) without being instructed to do so. If for whatever reason an ATSU cannot locate you on radar despite the fact you are squawking, "resetting" the squawk may enable them to do so. This is what the instruction to "reselect" means.