I don't think we should remove "usually" from the intro description to menubar.

Reason being that while a menubar can be permanently visible, it may not always be. For instance, WIndows 10 auto-hides application's menu bars, or other native computer applications, or web apps, can have their menu bar temporarily hidden by the user, as well.

regarding the proposed update to menu, I do like the idea of briefly mentioning example use, e.g. as a context menu, submenu within a menubar, or invoked by button activation. Weaving that into the description of the role would be enough, in my view, to indicate that the menu is not always visible without having to overtly state so.

Interested to know other's thoughts on that.

I don't think we should remove "usually" from the intro description to menubar.

I understand and agree.

Nevertheless, I think it would be good to better explain the role menu in contrast to menubar.

For Windows, for example, there is the following, clearly understandable definition

ROLE_SYSTEM_MENUBAR: The object represents the menu bar (positioned beneath the title bar of a window) from which users select menus.
ROLE_SYSTEM_MENUPOPUP: The object represents a menu: a list of options, each with a specific action. All menu types must have role, including the drop-down menus which are displayed when selected from a menu bar; and shortcut menus, which are displayed by clicking the right mouse button.

(According to Core AAM, the ARIA role menu corresponds to ROLE_SYSTEM_MENUPOPUP and the ARIA role menubar corresponds to ROLE_SYSTEM_MENUBAR)

Just a quick note -- the Windows API documentation linked in the previous comment is MSAA (the legacy Accessibility API), and not the UIA MenuBar and Menu docs. I'm actually not sure when MSAA docs were last updated, but they generally may not be the best primary reference for ARIA role descriptions.

Even UIA's Menu and MenuBar docs are primarily focused on describing application menus (i.e. the top-level menu that exists outside the focus order and is accessed with Alt). That said, their use isn't confined to application menus, and neither description specifies visibility (neither permanent visibility for menubar nor toggled visibility for menu) as a requirement.

I agree with @scottaohara that mentioning example use cases for each might help make the difference more clear without creating unnecessary limitations on the roles.