The Julian legion rules give you this ability, plus allow medium and heavy units to move 2 hexes but not battle. The Marian Legion rules just give you the ability to throw pila, so the Marian rules are more restrictive.
The Parthians are all cavalry, which means that they have a lot of mobility.
From
http://ccancients.netMarius Legions RuleStarting with Aquae Sextae and all later battles in this expansion, Medium Roman Infantry units are armed with pilum and sword. These units are now missile weapon capable units and must follow the rules for Ranged Combat units. The unit has a range of two hexes and will roll 1 battle die when it holds or moves
only one hex.
Julian Legions RuleWhen the Julian Legions Rule is in effect, all Roman Heavy infantry (red square symbol) and Medium infantry (blue triangle symbol) units are armed with pilum and sword and will follow the rules for Ranged Combat units, per the Marius Legions Rule. The unit has a range of two hexes and will roll 1 battle die when it holds or moves only one hex. It may not use Ranged Fire if it moves two hexes.
In addition, all Roman Heavy infantry (red square symbol) and Medium infantry (blue triangle symbol) units may move one hex and close combat as normal, or may move 2 hexes and not battle.
