Survivability is a conceptual attribute of units used to determine vulnerability and to properly allocate Attention. Military Survivability is generally considered in three contingent tiers: Susceptibility, Vulnerability, and Recoverability (NATO definition).
Susceptibility is the probability of a unit becoming the target of an attack, and of being hit if targeted. It is affected by tactics, situational awareness and countermeasures. Vulnerability is affected by hit points and shields, and is considered as the probability of being disabled or destroyed by a hit. Recoverability is a unit's ability to repair damage (either structural or shield damage). This is inherent to some higher tier units, as well as being granted by veterancy. Units' recoverability can also be boosted by the use of Engineers, Air Repair Pads (for air units) and Engineering Stations.
Survivability is extremely important because it often dictates how necessary some unit requires the Commander's Attention. An ACU has a very high Survivability in the Early Game and requires little attention. However, in the Late Game the ACU has an extremely low survivability.
Survivability is a conceptual attribute and is subsequently never clearly quantified, but can be qualified for substantial benefit. Factors that influence Survivability include the unit's defensive attributes such as Acceleration, Speed, HP, and Weapons with which to defend itself. A Galactic Colossus typically has extremely high Survivability due to its incredibly high HP and powerful weapons with which to defend itself. It can also submerge itself, further decreasing its vulnerability and thus increasing its survivability.
However, Survivability is most strongly influenced by the Enemy. The Enemy's capability to destroy a particular unit and the Enemy's perceived importance of mentioned unit typically has the largest impact on a unit's survivability. For example, a Serenity has more HP than a T1 Pgen, can move, and has a weapon to defend itself. However, it's Survivability is lower and thus it requires more attention. This is because the Serenity possess significant threat to the Enemy and is expensive to build and difficult to deploy, and only useful near enemy assets, whereas a T1 Pgen is inexpensive and possess no direct threat to the Enemy, and is typically built far from enemy assets, subsequently increasing its survivability substantially.
Survivability, as an end result, is effective in determining how much attention should be paid towards controlling a unit, and how much should be expended in it's defense, should the Commander require the unit to survive. For example, a Striker had a lower survivability in the early game than a UEF ACU, but a single striker is not particularly critical and it's loss is of little significance, where as the ACU is the focal point of battle, and thus requires a greater allocation of attention.