The underscore become needed since RouterOS command tree overlapped and also used type as an argument to RouterOS command recently. This messed by the JavaScript for a bit — since it no longer knew if was a arg or cmd... And why _type got used in newer inspect.json's.
But when _type change, never changed desc is used.
Mainly for constancy, the _desc should be used to connote some inspect metadata – i.e. not a potential child. See #10 - where there could more "metadata" subitems.
NOTE
As long asdesc does not overlap with any "arg" to RouterOS commands, there is no issue.