he means, if you want to attatch the rear brake resevoir to the correct mounting point you will have to remove the seat unit I.E the bodywork that you and a pillion passenger would sit on as the resevoir bolts to the seat subframe.

or you could buy a nice little HRC one from this very website
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MC21SP - Roth-tax
CBR400 - track
MC21R - sold
MC21SP - sold