The term is used to refer to any Theravada monks not within the Dhammayuttika Nikaya, the other principal monastic order.
As such, most monks in Thailand belong to the Maha Nikāya more or less by default; the order itself did not originally establish any particular practices or views that characterized those adhering to its creed.
There were in reality hundreds of different Nikayas throughout the Thai areas that were lumped together as the "Maha Nikāya".
A separate supreme patriarch for the Dhammayuttika Nikāya was appointed by King Norodom.
The previous national supreme patriarch then became the titular head of the Cambodian Maha Nikāya.