J is the most expensive business class fare, there are others whats called sub classes, of F/J/Y, those are as mentioned above, First/Business/Economy.
If you google search, you will find them.
Note, too that not all the codes are used by the airlines, some airline will use some alphabets, some airlines, will not use them.
Not all airlines have premY.
PNR is passengernamerecord, alphanumeric, ie, BB34JQ, usually if you mention that when calling an airline, they can use their booking system to bring up your booking, just by you quoting those 6 alphanumerics.
On a code share, sometimes, what one airline in that code share will use for PNR, another airline will have a different PNR code.
Airlines sometimes use the AlphaBravocharlieDelta code too, so if you can spell WhiskeyOscarNovemberGolf, its a surname, not mine.
If you wan to really learn, you will need a spreadsheet.
EK-Emirates
EY-Etihad
QF-Qantas
NZ-(Air)New Zealand
VA-Virgin Australia
These are ICAO codings, but there are also 4 letter codes.
And then there are also airport codes, AVV for Avalon, MEL for Melbourne (Au), don't confuse it with MLB for Melbourne (US)...
You don't have to worry about premY, (not prep economy), as 99.9% of Aust domestic flights don't have this.
Its only 2 flights, across the QF (Qantas) Au domestic sector that has this, and unless you choose to book it, you won't get to use it.
Its only on QF9/QF10, between MEL and PER, (learn these too), and PER and MEL, where you fly as a ((domestic passenger on an international flight)), on QF, where you need either a photo id or passport to fly this flight on the QF 787.
Some of us on here, make it a point to fly that plane/flight, a lot of people find it troublesome to go through immi and int security screening just to fly from PER - MEL/MEL - PER, adding more time.
But as these flights go through the MEL T2 int terminal, and the PER T3 QF int terminals, some of us get our kicks from that.
Give it time, one day you will get to know a tonne, Traccy!
Oh, most important, DYKWIA is DoYouKnowWhoIAm, when business class travellers in the premium boarding lane, at the gate, speak up when economy gets to board the plane first.