keyImGuiKey logical key identifier, not a platform-native key code.
One common use case would be to allow your items to disable standard inputs behaviors such as Tab or Alt key handling, Mouse Wheel scrolling, etc. e.g. Button(...); SetItemKeyOwner(ImGuiKey.MouseWheelY); to make hovering/activating a button disable wheel for scrolling.
Reminder ImGuiKey enum include access to mouse buttons and gamepad, so key ownership can apply to them.
Many related features are still in imgui_internal.h. For instance, most IsKeyXXX()/IsMouseXXX() functions have an owner-id-aware version.
Call SetItemKeyOwner(key) to assign the key to the last item when that item is hovered or active. The lower-level native ownership API is not exported.
Assign input ownership to the last item.