Mount & Musket is a very popular mod for Mount&Blade: Warband. The mod is multiplayer-only and has no singleplayer campaign. You can select historically accurate troop classes from the following factions: Russia, France, Great Britain and Prussia.
After the great success of the mod, fans began to organize a regimental system. A regiment is an online group that organizes special training, linebattles etc. Most regiments are by invite only but the best way is to usually ask someone in the regiment to invite you.
After regiments also became a popular part of the system, people decided to organize massive events in which players will form formations such as infantry square, column etc. but most commonly a line. most regiments will have a linebattle once a week but some of the more intense regiments have more.