Perhaps I can help,
SelCal, Selective calling, is so you do not have to listen continuously to the staticky HF radio. It is comprised of 4 tones, designated by letters. Ours are JLCR and CRAH. These letters or tones are assigned by an association who's name escapes me for the moment. After the tones you will hear 'N1234 answering SelCAl'. This establishes 2 way communication between the aircraft and whoever the controlling agency is. Sometimes it is for a reroute, a change of altitude or a center weather advisory.
In answer to your first question, airliners are required to report their fuel on board with their position reports. Hence; 'fuel 26 decimal 2' is 26,200 lbs of fuel. All jets measure fuel in pounds, but we purchase it in Gallons or Liters. The minus five two is the SAT, Static air temp, a negative 52 degrees Celsius. This is included with a MET report for winds aloft and turbulence. It's a PIREP out over the ocean and gives the forcasters current information to see how well the forcasts are holding up.
Hope this helps, for more info on the Atlantic Track System try this link!
http://www.nat-pco.org/nat/CurrentOpConcept.pdfRay