A unit entirely with Light Troops can perform Reform Move Manoeuvres up to twice its Mobility (instead of the normal half Mobility). If, while doing a Reform Move Manoeuvre, its centre moves further than its Mobility, the move is considered a March Move for the purpose of rules triggered due to Marching (e.g. Dangerous Terrain Test and, if Undead, being restricted from performing a March Move, etc.).
For the purpose of shooting, this is still a Reform Move. I.e. the unit can shoot if moving further than its Mobility
Note: this rule has no effect on other Move Manoeuvres, including March Moves
A unit containing Light Troops is always considered to have zero Proper Ranks; ignore Attachable Models with Light Troops joined to a unit for this purpose, unless the unit consists of only Attachable Models.