Effects (as used by items, spells, and EFF files) have a timing mode field. This field determines when the effect will occur, how long the effect will last for, and how creature stats are affected.

The engine stores two sets of stats for each creature, the 'base stats' and the 'modified stats'. Base stats are stored in the CRE file (or are constants, set when the game is loaded). Modified stats are determined at run-time by examining the base stats and applying any relevant effects. Scripting actions generally affect base stats, while effects can affect either base stats or modified stats.

Permanent effects (timing mode 1) usually modify base stats (when appliable) and then go away. Permanent effects (timing mode 9) modify actual stats and stay attached to the creature. Timing mode 9 effects are not removed by Ctrl-R and resurrection, timing mode 1 effects (that do not modify base stats) would go away after Ctrl-R or resurrection.