The style guide linked in the document seems to refer to the "6 types of British Nationalities", but fails to list them out.
Of the 6, the most quirky seems the "British Subject":
"You became a British subject on 1 January 1983 if, until then, you were either:
a British subject without citizenship, which means you were a British subject on 31 December 1948 who did not become a citizen of the UK and Colonies, a Commonwealth country, Pakistan or Ireland
a person who had been a citizen of Ireland on 31 December 1948 and had made a claim to remain a British subject.
You also became a British subject on 1 January 1983 if you were a woman who registered as a British subject on the basis of your marriage to a man in one of these categories. "
Of the 6, the most quirky seems the "British Subject":
"You became a British subject on 1 January 1983 if, until then, you were either:
a British subject without citizenship, which means you were a British subject on 31 December 1948 who did not become a citizen of the UK and Colonies, a Commonwealth country, Pakistan or Ireland a person who had been a citizen of Ireland on 31 December 1948 and had made a claim to remain a British subject.
You also became a British subject on 1 January 1983 if you were a woman who registered as a British subject on the basis of your marriage to a man in one of these categories. "