The management plane provides a single API entry point to the system, persists user configuration, handles user queries, and performs operational tasks on all management, control, and data plane nodes in the system.
For NSX-T anything dealing with querying, modifying, and persisting user configuration is a management plane responsibility, while dissemination of that configuration down to the correct subset of data plane elements is a control plane responsibility. This means that some data belongs to multiple planes depending on what stage of its existence it is in. The management plane also handles querying recent status and statistics from the control plane, and sometimes directly from the data plane.
The management plane is the one and only source-of-truth for the configured (logical) system, as managed by the user via configuration. Changes are made using either a RESTful API or the NSX-T UI.
In NSX there is also a management plane agent (MPA) running on all cluster and transport nodes. Example use cases are bootstrapping configurations such as central management node address(es) credentials, packages, statistics, and status. The MPA can run relatively independently of the control plane and data plane, and to be restarted independently if its process crashes or wedges, however, there are scenarios where fate is shared because they run on the same host. The MPA is both locally accessible and remotely accessible. MPA runs on transport nodes, control nodes, and management nodes for node management. On transport nodes it may perform data plane related tasks as well.
Tasks that happen on the management plan include:
Configuration persistence (desired logical state)
User management -- role assignments
Background task tracking