IRS

Both mobs and PCs can be immune, resistant, and/or susceptible (IRS) to various physical and magical affects. In general, the heirarchy is:

Immunity > Resistance > Susceptibility.

That is, immunity supercedes resistance which supercedes susceptibility. So if a PC is flagged to be both resistant and susceptible to fire, then that PC is actually resistant to fire. The susceptibility to fire is overridden by the resistance.

In terms of damage

Immune = zero damage taken
Resistant = half damage taken
Susceptible = double damage taken

In terms of spell affects:

Immune = immunity to that spell's affects.
Resistant = two saving throws vs. that spell's affects. Susceptible = will always fail a saving throw vs. that spell's affects.

NOTE: Mobs (but not PCs) may also be immune, resistant, and/or susceptible to weapons with given HITROLL bonuses.

See also: MOB IRS, RESISTANCES