Doxygen has three mechanisms to group things together. For RT-Thread API documentation, we use 'topics' to create new page for each group. On HTML browser, the topics pages are shown under the "Modules" in the treeview on the left.

To define a group, we use @defgroup command. The group name should be prefixed with a "group_", and the group name should be unique. We can define a group anywhere, not necessarily in the same file as the members of the group. Generally, we define a group in a header file.

To make an entity (structure, function etc. or another group) a member of a speicific group, we can use @ingroup command and put the @ingroup command inside its documentation block.

To avoid putting @ingroup commands in the documentation for each member you can use @addtogroup and group members together by the open marker @{ before the group and the closing marker @} after the group.

See documentation/0.doxygen/example/include/groups.h for example.

More information can be found in the Doxygen manual: Grouping.