Why Do We Recite The İnvocations Of Salli And Barik İn The First Sittings Of The Sunnah Of ‘Asr And ‘İsha Prayer And Recite Subhanaka At The Beginning Of The Third Rakat?

Why do we recite the invocations of Salli and Barik in the first sittings of the sunnah of ‘asr and ‘isha prayer and recite Subhanaka at the beginning of the third rakat?

As Hanafis generally consider every two rakats of nafl prayers as a separate prayer except the first sunnah cycles of dhuhr and Friday prayers, the first sittings are considered equal to the last sittings. This is why we recite duas of Salli and Barik in the first sittings of nafl prayers that are established before fardh cycles of ‘asr and ‘isha prayers. Recitation of Subhanaka in the following rakat after standing upright from the first sitting also results from this. (Bilmen, Ilmihal, p. 141-142)

Source: Presidency Of Religious Affairs The Turkey, High Board of Religious Affairs FATWAS