The autoload in the NetBeans module system world means that certain module is turned on when needed. In contrast to regular modules which require some manual action (user locating a module and activating it) to turn them on, the autoload modules are opaque for users and managed solely by the infrastructure. As soon as there is some module needing (by dependency or require/needs token) an autoload module, the infrastructure enables it.
In the OSGi mode, the meaning of autoload remains as close as the original meaning. The bundle JAR is registered to the underlaying OSGi framework, but it is not started until there is some NetBeans module which would depend on the bundle. If there is no such NetBeans module, the OSGi bundle remains in installed state (or resolved?).
The autoload mode is most suitable for OSGi bundles which shall be left in the full control of the OSGi container. For example there can be other OSGi bundles which may later start such bundles. Such enablement is outside of control of the NetBeans module system and is temporary (on subsequent launch of the system the module remains autoload).