You can purchase iOS apps by volume license. Individual app licenses are represented by 12-character redemption codes. Redemption codes can be made available to all of your managed iOS devices, or to a subset of them. Redemption codes apply only to iOS apps that are non-free and that were imported as external, Apple iOS Managed Apps.
Redemption Codes are assigned to user-configured containers called Redemption Pools. When created, a Redemption Pool is assigned to a Management Extender. After a pool is created, redemption codes are entered into the pool. iOS devices, which are managed by the Management Extender that the pool is assigned to, can be provisioned to use the codes in the pool.
To be able to perform the tasks Prompt Install Apps and Recommend iOS Apps on non-free iOS apps, the app must have available redemption codes that are assigned to it.