The infantry units line is intended as a upgrading path. So the start unit for units armed with fusil is the Arquebusier. After the discover of a specific Tech it can be upgraded to Musketeer; after the discover of another specific Thech it ca be upgraded to Fusilier and so on: Line Infantry 1780, Line Infantry 1810 etc.
The same thing for the others type of units, Pikeman, Light Infantry, units armed with sword at the start of the Mod...
For cavalry units we can start with Armored Cavalry that can upgrade to Reiters that can upgrade to Dragoon, then Dragoons 1670 and so on.
We have also Cuirassiers, Line Cavalry, different kind of Light Cavalry (Ussars, Lancers, Uhlans etc.) all with a proper upgrade path and specific Tech.