I guess you've never heard it called Plate Mail, Field Plate Mail, Full Plate Mail. You might be referring to Chain Mail and restricting the term 'Mail' to Chain armor, but the reality is, Mail just means 'Armor'. Could mean 'metal armor' I suppose, considering it would be strange to say you were wearing Leather Mail.
OK, thanks to the internet being a thing now (and not in my childhood when I was learning everything through inference in terminology alone along with 'meh' dictionaries) I've done some research into this at this prompting and have to correct myself.
Apparently,
"Sometimes
chain mail is just
called mail, from the Old French maille, "mesh.""
So the term 'mail' is really just referring to chain 'mesh' armor. Thus chain mail is an armor and plate mail is an armor BECAUSE it includes chain in the design. This is an interesting distinction I'll be sure to consider in the equipment mod. I always understood it as I originally stated in the quote here and yeah, ok, I'm wrong.
That said, to say a unit is a Mailed Cavalry is non-specific as to just how heavy that armor actually is isn't it? Or would you say that unless 'plate' is specifically added before the term 'mail' that it defaults to simply meaning chain-mailed? (EDIT: Some later found sources do assume this.)
Might have to change the naming so as to not be so specific just to "Heavily Armored Cavalry" or Plate Mailed Cavalry, or Plate Cavalry, since the assumption on that unit should be that it's in the heaviest armor it can get its hands on - even rivaling knights to the point that if they were dismounted they might not even be very capable of fighting on their feet as the weight is so burdensome. Full Plate Cavalry perhaps.
The Knight unit is assumed to be in something more plate mail oriented, perhaps as some call it field or half-plate. A bit more suited to melee fighting when dismounted.
The intended distinction is interestingly detailed here:
"
The highly developed weight distribution that became possible with the era of Articulated Plate (What our Knights would tend to wear)
was simply amazing. A warrior clad in a suit of properly made Articulated Plate would have been able to do cartwheels in his armour. Each plate balanced perfectly to a part of his body. This effectively dispels the myth of knights in armour falling off their horses and not being able to stand up. Such stories must have developed from certain examples of Tournament Plate Armour. Tournament Plate was specially designed to take the incredible impacts of jousting, so that it was very, very heavy. Such armour would have never been worn outside of a tournament environment. Again, it is important to compare the armour to the opposing arms and warfare techniques of when it was used."
The unit we're discussing would wear something closer to this heavier tournament plate as they aren't really intending to dismount to fight unless they are forced off.
Further research on the wikipedia pages you linked shows I wasn't entirely wrong either:
"After the fall of the Western Empire, much of the infrastructure needed to create
plate armour diminished.
Eventually the word "mail" came to be synonymous with armour"
No doubt that has led to a wide potential range of conclusions as to what the term means in terms of what it's defining that has led to a great deal of ambiguity. Thus my original presumption that 'Mailed Knight' was simply referring to a more heavily armored version of a knight wasn't far off track and potentially exactly what the original naming was intending.