Serial numbers of software; unlock codes which, when entered, change an unregistered (and limited) demonstration version of a program into the full version.
To find hackz, crackz and serialz, tryentering these terms into Google. ;)
Simultaneous oral sex. Can be practised by M+M, M+F or F+F.
The reference is that the numerals 6 and 9 are inverted versions of each other, reflecting the physical position that the couplemust adopt when engaged in this sexual practice.
Back in his room, he discovered she was a ladyboy. But as he'd already paid, he thought he'd better make the best of it. It was the best head he'd ever had.