A unit's Decommission bonus (also known as Bounty value within game-data files, and here referred to as Refund amount) is an amount of resources that the player may gain when non-Hero and non-Siege units are no longer wanted. The reason this feature is a "bonus" is because the player is being refunded for a unit regardless of how long the unit was used for battling the enemy. In fact, refund amount will increase as the unit's level increases - but the amount varies for different units in different factions.
Decommissioning is done by selecting any amount of unwanted infantry, and then right-clicking on any of the player's Fortresses or, if the player is based in the stronghold of a Fortress-map, by right-clicking on the Keep. The decommissioning feature is ideal for when the player's command point amount is consisting too much of battalions that are at low health and cannot heal, as is the case with units of all Evil factions.
As each decommissioned individual enters, a very minimal refund to the player's resources will be shown. For example, the refund amount for each individual Easterling pikeman (shown above), when the battalion is at Level 1, is two resources.
For the forces of Mordor alone, there is one special condition to refund-locations: Orc archers and warriors can be decommissioned at a Slaughter House, for a larger refund than if sent anywhere else.
In The Battle for Middle-earth II, the factions of Men of the West and Dwarves have ways of increasing their units' decommission bonus, which is through the construction of Hall Expansions and Dormitories, each of which increase the bonus by 25%. (On Fortress-maps, this will only work if the faction is not based within the stronghold.)