As observation model of PPP-RTK is not full-rank, interpretation of such corrections as satellite clock, code bias, phase bias, and STEC can be different among service provider in general. Therefore, interoperability between different service providers and users has been a challenging problem. In this paper, by setting certain constraints on the code bias of two reference signals and STEC of a pivot receiver, we mathematically prove that correction data generated with combined observation equation can be applied to positioning algorithm with uncombined observation equation. Whichever uncombined or combined model is applied in the service provider, the same positioning algorithm can be applied in the user side. With the proof of this, we support the standardization of SSR messages in RTCM.