1 b.2.c result

What does it mean by electronic assembly type?

Electronics Forum | Tue Nov 18 01:21:14 EST 2008 | kywl

Hi all, I am referring to IPC 1720, and i notice that there's electronic assembly type 1A, 1B, 1C, 1X, 2B, 2C... 2Z... can anyone please help explain what do those numbers mean? I have no clue at all... Thanks!