This is a Shimano Deore XT BB-MT801 (BSA Hollowtech II) bottom bracket.
It comes with three 2.5mm spacers. Note that one spacer must be installed on the right side anyway (68/73).
As far as I understand, the 2.5 mm place was originally intended for certain types of front derailleurs. If you are using a different type of front derailleur, or none at all, the space must be filled with a spacer.
