The compressed air feed to the mask goes through a filter, fine filter, coalescing filter and then an activated carbon filter which is on the painters belt (this may be what was meant by 'charcoal filter').
The filters are to remove any oil that is carried over from the compressor because you don't want to breath it in and you definitely don't want it in the paint. The activated carbon filter removes smell.
The company I work for supply the filters/coalescing filters but the activated carbon filter is a proprietary unit used in the spray industry with simple drop in cartridges.
ITW Finishing Technologies Australia
Colin