The boot nipple is a place on the rubber boot that aligns the rubber boot in alignment with the cylinder so that the carburetor fits on the bike correctly without gaps in the sealing of the unit. The rubber boot is the black rubber piece that fits between the cylinder and carb on both front and back cylinder.
If you haven't messed with your carbs yet then right click on the pictures and download them or "save as" so when you do mess with them you will understand what we are talking about.
This has been a very aggravating piece to find when reassembling the carb to the engine.
Grace and Peace to you!