Android Enterprise Agent and Plugins are for Android-Enterprise platform.  The agent is generic in the sense that all the many tens of supported OEM brands/models use the same agent to support a basic feature set defined by Google.  OEM specific features are provided by the plug-in available for each brand/model.

Android - OEM Specific Agent and Plugin are for Android Plus platform.  Each agent is specific to a brand or a device series of a brand.  The feature set supported by each Android+ agent can differs significantly, principally based on what the OEM device brand owner wants to support in its device firmware.   Android+ plug-in has recently been added to support some enterprise-grade features for particular OEM  device brand.