yehudayona: The Mechaber has it as an issur to mix in matza as it can accelerate chimutz, Rama just says we don’t use it at all because it is ‘like kitniyot’,without elaboration. Mishna Brura there give the reason that is is davar demidgan, as in the older texts, midi demidgan. This is the broad brush of kitniyot, as it subject to many definitions. Some that I have heard or seen:
Handled like grain
Harvested like grain
Looking like grain
Legumes
Substitution for grain
Growing in pods or in clusters
As you can imagine this can broaden the range of what is exactly kitniyos.