The B15 is the platform the car sits on.....
for instance:
91-94's B13's
95-99 Sentra's and 200sx's B14's
2000-2002 Xe,Gxe,Se,Se-R,Se-R SPec-V's B15
And as far as when we reffer to the SR20DE that's simply the 2.0L engine that has been featured in Sentra's and 200SX's since 91-2001...Hope this clear things up a bit...
