The proposed categories for renderables are:
Components (built of smaller elements)
Layouts (structured arrangements of smaller components/elements/layouts)
The names of these categories needs to be agreed on, and docs created to describe each category.
An (early) list of renderables is defined here: http://docs.moodle.org/dev/Render_library_specification#Initial_thoughts_on_renderables_and_what_we_should_provide_.28Draft.29
This list needs reviewing and categorising.