use_space

It’s a toggle button.

This is one of the most important feature.

When on(green):

will pop different set of menus based on context/area like 3D_VIEWPORT, GRAPH_EDITOR, TEXT_EDITOR etc.

Note

Blender’s quick shortcut button (shortcut Q) works this way by default, meaning different areas have different entry based on context.

When off(reddish):

will pop common set of menus for all context/area

Areas

CLIP_EDITOR
CONSOLE
DOPESHEET_EDITOR
FILE_BROWSER
GRAPH_EDITOR
IMAGE_EDITOR
INFO
NLA_EDITOR
NODE_EDITOR
NODE_EDITOR_PATH
OUTLINER
PREFERENCES
PROPERTIES
SEQUENCE_EDITOR
TEXT_EDITOR
UV_EDITOR
VIEW_3D

Note

In Future there will be even more categories, based on modes (like edit mode, object modes etc), use_context is more appropriate name for this.