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 Data Center 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 Data Center UI.
In NSX there is also a management plane agent (MPA) running on all controller cluster and transport nodes. The MPA is both locally accessible and remotely accessible. On transport nodes it may perform data plane related tasks as well.
Tasks that happen on the management plane include:
Configuration persistence (desired logical state)
User management -- role assignments
Background task tracking