Ordnance is a limited or slow-reload explosive weapon for a Pilot or Titan. Pilots are limited to 2 ordinance ammunition (3 with the Explosives Pack Tier 1 Pilot Kit equipped), although this can be restocked by getting in a Titan and hopping out again. Titan ordinance weapons have "unlimited" ammunition but are substantially slower to reload than any of the primary Titan weapons.
There are four different types of ordnance available for both Pilots and Titans.
Frag Grenade - Deals explosive damage to all enemies in a small radius
Arc Grenade - Deals less damage than the Frag Grenade but also temporarily scrambles the visual systems of any Pilots and Titans caught in the blast radius.
Satchel Charge - Deals more damage than the Frag Grenade and in a larger radius. Can be thrown or stuck to any surface. Satchel Charges must be detonated remotely by the player once thrown or placed.
Arc Mine - A proximity mine that can be stuck to any surface. Deals more damage than the Arc Grenade (but less than a Satchel Charge) across a wider radius and has an increased duration of visual disruption.