Resistance to magical effects on FR is handled by properties.
In general equipping an item that confers resistances adds a static
property to the affected player.

The types of resistances available are listed under "reserved".

A few guidelines. Resistances up to 10% can be added without approval
up to 25% is generally acceptable without penalties.
Above 25%, an appropriate succeptability should be added as well.
I.e. +50% fire and -50% cold.

