A direct fire unit is a unit that has weapons that follow either a linear or low-profile trajectory. These are opposed to indirect fire units, which fire weapons with greater range and higher trajectories. Their low-profile trajectories can interfere with their ability to fire over obstacles like terrain and walls, but the shorter time between firing and hitting their target helps hit moving targets.
A direct fire unit can hit land units, structures, floating units, hovering units and aircraft that have landed on the ground.
For a complete list of direct fire units, see the direct fire units category.