The iprp_poison.2da file is the .2da file that defines the on-hit poisons available in the game. This is basically a way to associate poison names (e.g. "1d2 Strength Damage") with abilities, so the customization possibilities are limited. This file is used (only) when referred to by the "TableResRef" column of iprp_paramtable.2da, and it should be referred to by (only) row 10 of that file.