Some good info here that has answered some questions I had also. Thanks. Another one - has anyone experienced the issue where when you transit from a CPDLC supported FIR, to an FIR that doesn't support CPDLC and then subsequently back into a CPDLC supported FIR, that the FMS remains "locked" on the initial CPDLC FIR? I've found the only way to "unlock" the initial FIR is to select ATC CPDLC OFF (probably not the correct term, can't remember what the available selection actually was) and then re-enable it to allow login to the new FIR. Anyway, that was my workaround. Gotta say, it is the duck's nuts, simply awesome. Out of PHNL to YSSY - last time I spoke to someone was at TOPC until I was on VHF with Brisbane over Lord Howe Island. Never loved HF...
(By the way, I'm referring to GLEX)