How to use 'bl_keymap'?

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Hello!
Im creating a addon for blender, and found a line of code in blender official python sample:

    bl_keymap = (
        ("view3d.select_circle", {"type": 'LEFTMOUSE', "value": 'PRESS'},
         {"properties": [("wait_for_input", False)]}),
        ("view3d.select_circle", {"type": 'LEFTMOUSE', "value": 'PRESS', "ctrl": True},
         {"properties": [("mode", 'SUB'), ("wait_for_input", False)]}),
    )

I know it’s assigning a hotkey to a workspacetool.
So the problem is I cant find any infomation about ‘bl_keymap’ in blender api.
And another line of code ‘def draw_settings(context, layout, tool)’ is the same problem.
How can I get infomation about these code?