The designated bilingual regions are listed in a Treasury Board circular dating from 1977, so these regions were determined quite a long time ago.
The term "designated bilingual regions" may be a bit startling. We have to understand that they are defined as such for the purposes of the language of work. So this is only about work within the institutions. It is not talking about the language for services offered to the public. It is talking about the employee's right to require that they work in their preferred language.
As I said, designated bilingual regions are found more or less all over Canada for the purposes of the language of work. The entire province of New Brunswick is one, but also certain regions of Quebec and in eastern and northern Ontario, and of course the national capital region.