What is the definition to A, B, C, D, E and F cores?

Per SDI-100-1991 the cores were listed as follows:
A – Kraft Honeycomb
B – Polyurethane
C – Polystyrene
D – Unitized Steel Grid
E – Mineral Fiberboard
F – Vertical Steel Stiffeners

These are now archaic references and should be treated as such.