@No Apologies
Metropolitan is where you have one city that contains at least about twenty congregations or more in its catchment area (say any congregation within ten miles of the cart witnessing places).
A Circuit Overseer is responsible for the Metropolitan sites within that area - usually close to shopping malls, train/bus stations, business districts etc. He then appoints a lackey "up and coming" elder or two to oversee it and arrange schedules of (usually) pioneers and sometimes elders and their wives from various congregations to cover "trolley/cart shifts" of three publishers together.
Those chosen to go on Metropolitan come from those approved by each congregation service committee and filtered through the CO and his lackey assistant.
"Normal" cart witnessing is that which is arranged by each congregation in its own shopping areas etc but which shouldn't conflict with Metropolitan. The ones allowed to do congregation cart witnessing are approved by the local Body of elders and usually include anyone who is not any or all of bonkers/smelly/dressed like a sack/tourettes and the like.
In the UK a city like London would have several Metropolitan circuit cart set ups; somewhere like Birmingham/Glasgow (1 million pop) would have two or three. Cardiff/Bristol/Edinburgh (350/400,000) might have one each. As in one circuit arrangement with two or three sites in each city.
Smaller cities such as Plymouth/Newcastle or Southampton might just have one or two sites in each and only half of the congregations in a circuit providing labour.