After a word denoting sources of revenue there is the affix ṭhak in the sense of 'come thence'.,
The word आय means what is the share appropriated by the lord. The source of आय is called आयस्थान ॥
This debars अण् and छ ॥ Thus शौल्कशालिकः 'what is derived from custom house'. आकरिकम् 'revenue derived from mines'.
The plural in the sutra prevents Svarupavidhi 1.1.68.,