What does TR mean?

IE: (2,500¥ + 500¥ * TR)

table rating. it's a tool used for shadowrun missions to make their missions scale better; a shadowrun team with a lot of karma (and presumably gear) will have a higher table rating and therefore will require tougher opposition, but can also expect to get paid more because they are more experienced (and due to the way table rating works, the run is also a more difficult one too)

Real quick the math is to my knowledge layed out as 2,500 + (500 * TR) with math I belive you do the bracketed numbers first if you didn't know.

So say your running a table with players that raise it to level 3 you would do 500 times 3 = 1,500 + 2,500 = 4,000

TR = Threat Rating

Doesn't matter how you write it... Order of Operations does multiplication first, then addition...

so 2,500 + (500 * TR) = (2,500 + 500 * TR)

In either case... this is definitely a formula to calculate mission payout from SRM. It looks like the formula for the first SR4 mission's payout.

I thought I'd mention that as of SRM03, the players at the table choose the TR for the scenario themselves, rather than going automatically by Karma.

In that case. TR = # of PC to kill...

Aye, but it doesn't equal (2500+500)*TR. That subtle difference can be very important. In context, it is more likely that they'd have just written 3000*TR if that was what they meant, but you never know.

Where can find this in the main book?

It not in the main book, it's in the first part of the SR4 mission available for download.