Let me give my definitions. You can tell me if you agree or disagree, and then we can work from there.
Generic Unit-Line: A unit line whose base unit is available to over 75% of civs. As an example, let’s take the militia line. The base unit is militia, which is available to all civs. Therefore, this belongs to the generic unit-line. Another example is the Mangonel, which is also available to all civs.
Generic Unit: A unit which is available to over 75% of the civs, or is part of a Generic Unit-Line which has no variation. In cases where the Generic Unit-Line does have variations, the unit is generic if it is the standard continuation of the unit-line. That is, if the unit is present in the unit line for the overwhelming majority of civs it is available to.
This is important. So, trebs are a generic unit, because they are available to all civs. However, they cannot be upgraded. Therefore, they are not part of any generic unit-line. Siege Onagers are available to less than 25% of civs, but they are still generic. Now, consider Winged hussars. In the overwhelming majority of cases, the scout line end in hussar, if hussar is available. Similarly, legionary is an upgrade to the militia line, but it is only available to one civ. That makes them non-generic. Same for imperial camel.
Unique Unit: A unit that is available to just one civ, but can be shared via UTs. Meaning, only one civ can produce them, in all standard game modes, and no data mods. There is one notable case here, which is the Kipchaks. They are shared as part of a UT.
Bonus Unit: A unit that is shared by one civ to another civ as a civ bonus. Imperial skirms, condos, and genitours belong to this category.
I am putting these in a separate category because they are extremely difficult to classify otherwise. They are certainly not generic, because most civs won’t have access to them under most circumstances. They are not unique, because everyone on the same team gets to produce them. They are also kinda weird in terms of flavour.
Does it make sense that meso civs, south asian civs, and african civs get to produce condos because they have allied italians? Why are no other units shared? Why is one of this an upgrade to a generic line, while others are their own units? Why does only one of these have an elite upgrade?
Regional Unit: Everything that does not belong to any other category I listed. So, these are not generic units, and not unique units.
I hope that answers your question, feel free to ask for clarifications, or disagree with any of my points.
Yeah, this is the problem. It feels like Knights are in a weird place right now. They were decisively generic a year ago, and suddenly kinda not. So, I’m not sure what to do with them in terms of theory crafting, and even gameplay-wise.
What? No. I’m avoiding them because unique units are difficult to classify in the variety available from a building. Let’s say they introduce a new civ which has 33 new units in the barracks. Does that now make the barracks the most diverse building, when everyone else only has 2 units?
A unique unit, by definition, is a unit defined by a single civilization. It does not matter if it is produced from barracks, or archery range, or stable, it does not count towards how diverse that building is, for the entirety of the game.