The augment suṭ is placed before ka when sam , pari , and upa are followed by the verb kṛ and the sense is that of ornamenting.,
Thus संस्कर्ता, संस्कर्तुम्, संस्कर्तव्यम् ॥ The म् of सम् is changed to anusvara by 8.3.5. So also परिष्कर्ता परिष्कर्तुं परिष्कर्तव्यम् ॥ The स is changed to ष by 8.3.70. So also with उप, as उपष्कर्ता, उपष्कर्तुम् उपष्कर्तव्यम् ॥ Why do we say 'when meaning to ornament'? Observe उपकरोति ॥ Sometimes सुट् comes after सम् even when the sense is not that of ornamenting : thus संष्कृतमन्नम् ॥,