When over ATS with MTI CLA it is used for Oceananic Clearance Acknowledgement.
B2 is part of the OCRA chain of messages
/PIKCLYA.OC1/CLA 1636 220610 EGGX CLRNCE 465
DLH404 CLRD TO KJFK VIA GOMUP
NAT B
GOMUP 59N020W 59N030W 58N040W 56N050W JANJO
FM GOMUP/1805 MNTN F360 M080
END OF MESSAGEDBF3
/BDOOCYA.OC1/CLA 0928 220616 ENOB CLRNCE 016
SAS945 CLRD TO KORD VIA GIKOR
RANDOM ROUTE
GIKOR 65N000W 66N010W 67N020W 67N030W
65N040W 62N050W AVUTI
FM GIKOR/0953 MNTN M082 F360
-ATC/AFTER PASSING AVUTI F
Example from VDL message 851451301
/PIKCLYA.CLA 1346 230213 EGGX CLRNCE 397 AAL155 CLRD TO KSEA VIA BALIX RANDOM ROUTE 61N020W 64N030W 66N040W 67N050W DARUB FM BALIX/1441 MNTN F360 M084 ATC/ROUTE AMENDMENT END OF MESSAGE
- CLA = Clearance Acknowledgement downlink message
- CLX = Oceanic Clearance uplink message
- FM = from
- MNTN = maintain
- EGGX = Shanwick oceanic FIR
- ENOB = Bodo oceanic FIR
Analysing message 851451301
<Sending station>.CLA<Message time in HHMM> <Message date in DDMMYY> <First oceanic FIR entered> CLRNCE <Clearance ID number> <Aircraft callsign> CLRD TO <Destination> VIA <Routing via NAT or RANDOM ROUTE including coordinates> FM <Oceanic entry point>/<Time in HHMM at entry point> MNTN <Cleared Flight Level> <Cleared Mach Number> <Remarks from ATC> END OF MESSAGE
Some messages have line breaks in, some do not, depending on the sending authority. Messages all take the same format however.