Complex sentence: It was, and is, used to describe territories in which the monarch exercises sovereignty, the phrase "His/Her Majesty's dominions" being a legal and constitutional phrase that refers to all the realms and territories of the Sovereign, whether independent or not.
Split into simpler sentences: It was, and is, used to describe territories in which the monarch exercises her sovereignty. The phrase "Her Majesty's dominions" being a legal and constitutional phrase that refers to all the realms and territories of the Sovereign, whether independent or not.