An Expeditionary Transfer Dock (ESD), formerly the Mobile Landing Platform (MLP), are designed to be a semi-submersible, flexible, modular platform providing the US Navy with the capability to perform large-scale logistics movements such as the transfer of vehicles and equipment from sea to shore. These ships significantly reduce the dependency on foreign ports and provide support in the absence of port availability. The class also houses a sub-class variant called the Expeditionary Mobile Base (ESB), formerly the Afloat Forward Staging Base (AFSB).

The ESD and ESB (Expeditionary Mobile Base) and are part of a new ship class added in 2015 with an E as a new designator, similar to the L-class amphibious ships, S-class submarines, A-class auxiliaries and more. These three E-class ships were previously listed as seabasing ships in the Naval Vessel Register.
https://news.usni.org/2018/02/22/2nd-expeditionary-sea-base-usns-hershel-woody-williams-delivers-navy