The affix khaś comes after the verb pac 'to cook' when the object in composition with it denotes a measure.,
Thus प्रस्थं पचति = प्रस्थम्पचा 'a pot having the capacity to cook a prastha of food'; so also द्रोणम्पचः, खारिम्पचः 'a kettle'.,
परिमाणे पचः
Adhyāya 3 · Pāda 2 · Rule 33