A secondary belt is used for carrying second-line carried gear holsters, pouches and sheaths that you want to be able to take off when necessary.
Most secondary belts have fastex quick-release buckles so as to be easily put on and taken off.
Some secondary belts have PALS webbing for malice clipping on accessory pouches.
Police carry most of their gear such as guns, knives, batons, tazers, pepper spray, handcuffs and radios on a second-line duty belt.