Center Bolt kit is used to secure a dock section on the J-Brackets. Drill a hole straight through the end of both sections between the J-Brackets and insert this bolt.
This will keep the dock section from lifting off the J-Brackets or sliding one way or the other. This should be within reach from above the dock so you can remove it in cold water before freezing.
Docks that are 6′ or wider should have two or more center bolts.
The video here features the center bolts when 2 sections join together on the J Brackets.