AARRRGHHHH!!
leure:
Welcome aboard!
To see the difference between any kind of unit, click over their NATO icons to open the unit detail window. There you will have all the statistics for the selected type of element (the building blocks of a unit).
Line and light infantry ara pretty similar. On real ACW the distinction was mostly in the name. Both are intended to figh in the line of battle, on divisions and corps.
Skirmishers: if you mean sharpshooters, they are very useful because of their special ability that improves the initiate of all the units on a division.
Put one on each division if you can.
Raiders, partisans and the like are different thing. Don't send them to battle as they are very weak. They are fast and sneaky and best used with the evade combat button pressed. Use them to infiltrate behind enemy lines, scout, destroy railroads, burn supplies and cause havoc.
Just be prepared to lose them pretty soon if they get caught up or run out of supply while on enemy territory. They are kind of "disposable"
Regards
PS: Use the forum search feature. it works great and you will find tons of info about neartly any doubt you might have.
Regards!
