The Undertow and EJB subsystems depend on certain abstractions provided by the clustering subsystems in order to support clustered deployments.
Hard-coding clustering subsystem dependencies within the EJB and Undertow subsystems creates a tight-coupling between the subsystems in question, and also makes configuring the various dependencies (or plugging in alternative dependencies) difficult. Defining a separate "configuration" subsystem to sit in between the EJB/Undertow subsystem and the relevant clustering subsystems simplifies configuration and provides a looser coupling between the subsystems.
The distributable-web subsystem was introduced to provide decoupling between the Undertow subsystem and the relevant clustering subsystems.
The aim of the distriubutable-ejb subsystem is to do the same for the EJB subsystem and the clustering abstractions that it depends on.