Skirmishing units, one stand per hex, cannot be shot at by enemy artillery or shooters unless in hex-to-hex contact. This contact usually only occurs when skirmish stands are unable to evade.
Missile troops cannot shoot through skirmishers which would be in effect in hex-to-hex contact with an enemy unit.
Can artillery shoot through skirmishers? This has been the subject ofsome debate, probably because of the role of skirmishing infantry in various sets of Napoleonic rules. We allow medieval artillery to shoot through skirmish screens at formed units without penalty. This is based on the belief that the gunners can see targets through the thin line of skirmishers. However, skirmishers can be very useful at shooting and disrupting artillery units which are usually C class.
Evade/flee and pursuit. Example:
If a unit of cavalry with a movement of 4 hexes moves 2 hexes into contact with a unit of Mongol light cavalry (movement 5), the Mongol light cavalry have the option of evading or fleeing. If they choose to evade, they move back 1 or 2 hexes and continue to face the enemy, in which case the enemy cavalry still has 2 of its 4 hexes of movement left, which it can then choose to use to pursue and make contact once more with the evading Mongols, or alternatively stay in its current hex.
When contacted, if the Mongol light cavalry chooose to flee, because it is a 'voluntary' flee move it costs 1 hex to about turn and then moves away upto 4 hexes. The enemy cavalry can then continue to to use up its remaining 2 hexes of movement in 'pursuit' or indeed any direction, but will probably not be able to make contact again with the original Mongol unit which it contacted.