The iprp_paramtable.2da file is the .2da file that defines the various tables used to interpret the "parameter 1" value of an item property. Each item property has a type corresponding to a row in itempropdef.2da, and the Param1ResRef column of that row is an index into this file (iprp_paramtable.2da). The referenced row from this file then gives the name of a third .2da file to use when interpreting the item property's parameter 1 value, which is an index into that third .2da.