The affix cha (īya) is optionally added to a stem ending in sthāna if it is compounded with something with which another is made to take an equal place.,
The word सस्थान means समानं स्थानमस्य 'whose place is the same\ or तुल्य or equal. Thus पित्रा तुल्यः = पितृस्थानीयः or पितृस्थानः (पितुरिव स्थानमस्य whose place is that of a father). Similarly मातृस्थानीयः or मातृस्थानः, राजस्थानीयः or राजस्थानः ॥
Why do we say \when it means equal to\. Observe गोस्थानम् \cow-shed\ अश्वस्थानम् \stable\. The word इति in सस्थानेनेति is for the sake of distinctness. The word सस्थान as a Bahuvrihi compound is the appropriate word to be taken here, and not when it is taken as a Tatpurusha compound like समानं च तत् स्थानं च ॥ The word चेत् \when\, is a conjunction : and joins the two meanings.
The rule 5.3.9 occurring between this rule and 8th rule, both of which are optional, must be taken to be a nitya rule, as explained above. द्वयोर्विभाषयोर्नित्या विधयः ॥,