EMP stands for Electro Magnetic Pulse. EMP is used to short circuit and temporarly disable vehicles and buildings. EMP cannot effect commander units or experimentals except when delivered as part of a death effect such as that of the Loyalist. Units that have this ability: Medusa (Cybran T1 Artillery) Loyalist (Cybran Siege Assault Bot; occurs on death) Shimmer (Aeon T1 Bomber), EMP Flux Warhead (Launched by Cybran Liberator, causes temporary EMP effect to any surviving enemies in blast radius), the Cybran T2 Air Transport, and the Cybran T3 SACU (when upgraded).
EMP AoEs pass through shields, both personal and bubble. However, if the EMP detonates on a large enough bubble shield, its radius at detonation may not reach the target. This is visible with smaller EMPs and the Parashield.
EMP weapons depend on Tier. A Tier 1-2 unit can stun up to Tier 2 units, but they are unable to stun T3. The T3 Cybran SCU can stun T3 units, including other SCUs. (This is changed in the FaF version, where SCU’s gain the ACU’s level of resistance.)
Only two items in game contain an EMP powerful enough to stun an ACU or an Experimental, the Cybran Loyalist's death weapon, and the Liberator nuke.
EMP Shorthand: Medusa/Shimmer/Cybran T2 Air Transport - Stuns T2 and lower.
Cybran T3 SACU / Aeon ACU - Stuns T3 and Lower
Liberator/ Loyalist - Stuns everything.
This list is for units only. Structures in general are resistant to EMP effects. Loyalists, SCUs, and ACUs cannot stun them. However, the Shimmer and Medusa can stun T1/T2 structures. The Liberator still stuns everything.
Units are able to continue building structures, repairing friendlies, and acting as engineers while under the effects of EMP. They can receive orders, but they cannot fire or move.
Factories and build-capable experimental units are able to continue building units while under the effects of EMP.
Duration wise, EMPs don't last long in game. A Cybran SCU with the EMP upgrade fires fast enough to permanently lock down and kill a Percival. Cybran RoF in general is high enough to freeze a single target entirely.
The Liberator Nuke, the strongest EMP, last about 14 seconds, and works at its outer damage range.
The Aeon ACU carries a Chrono Dampener, which stuns units around it. While not officially an EMP, it acts using the same mechanism, and can also stun T3 units, but not experimental units or other ACUs.