What's the relation between the Kushan Empire and the Sassanid Empire in Iran?

Kushano-Sasanian Kingdom (also called Kushanshahs,  or Koshano Shao in Bactrian, or Indo-Sasanians) is a historiographic term used by modern scholars to refer to a branch of the Sasanian Persians who established their rule in Bactria during the 3rd and 4th centuries CE at the expense of the declining Kushans. They captured the provinces of Sogdiana, Bactria and Gandhara from the Kushans in 225 CE. The Sasanians established governors for the Sasanian Empire, who minted their own coinage and took the title of Kushanshas, i.e. "Kings of the Kushans". They are sometimes considered as forming a "sub-kingdom" inside the Sassanid Empire.