After a stem ending in the word devatā being in the Dative -4th Case in construction comes the affix yat (yaṃ) in the sense of 'appointed for it'.,
The word तादर्थ्यम् has the same meaning as तदर्थ \for the sake of that\ 2.1.37. Thus अग्निदेवतायै इदम् = अग्निदेवत्यम् \offering &c, sacred to Agni\. So also पितृदेवत्यम्, वायुदेवत्यम् ॥,