Explanation (in the UK):
Special Metropolitan Witnessing is what is arranged at circuit level in a "big city". They typically choose areas near big shopping centres or railway stations to set up the carts.
This has been the preserve of regular pioneers from surrounding congregations appointed by the Circuit Overseer (or more accurately he rubber stamps those collated by the dumb schmuck elder that gets to do all his menial work - in return for a bit part on the assembly program of course)
Normal cart stuff is what any congregation organises outside the local shops or whatever and keen elders who want to do the same as the circuit overseer only use regular pioneers and maybe a few favourites like themselves and their wives if they want to bump their hours up. These are chosen by the Service Committee or the body of elders. More lax congregations let most of their publishers go on the carts unless they are really wackadoodle crazy.
Cardiff in Wales (pop 300,000) has these SMPW set ups and they have just had a letter read out saying that Elders can recommend good non-pioneers for this work.