A managed field is a CMP or CMR field that is mapped to the same database column as another CMP or CMR field. CMP fields mapped to the same column and CMR fields mapped to exactly the same column lists always have the same value in memory. For CMR fields that share only a subset of their mapped columns, changes to the columns affect the relationship fields in memory differently. Basically, the GlassFish Server always tries to keep the state of the objects in memory synchronized with the database.
A managed field can have any fetched-with subelement. If the fetched-with subelement is <default/>, the -DAllowManagedFieldsInDefaultFetchGroup flag must be set to true. See Default Fetch Group Flags and fetched-with in Oracle GlassFish Server 3.0.1 Application Deployment Guide.