WW_Somme represents the battle of the Somme, an infamous battle which took place during the later months of 1916.
WW_Marne represents the first battle of the Marne, the battle that halted the German advance into France in 1914.
WW_Devilswood represents the fighting around Delville Wood, during the battle of the Somme.
WW_Passchendaele represents the Third Battle of Ypres, more commonly known as Passchendaele.
WW_Bunker represents an unknown battle which probably would've taken place during the winter months of the war. It is unclear where or when this map is set, though it's safe to assume that it is on the Western Front, given that this is the location of all other maps.