For the purposes of recycling systems, "greywater" is the wastewater from showers, baths and wash-hand basins
A GWR system collects, processes and stores greywater for subsequent re-use as non-potable water, ie for toilet-flushing, clothes-washing machines, and irrigation.
RWH works best when there is a good match between the amount of rainwater that can be collected, and the associated demand for non-potable water. When much less water can be collected than can potentially be used, then GWR comes into its own. Multi-story hotels and hostels, for example, usually have limited rain-collecting areas (ie roofs), compared to their occupancy; conversely, there is a very good match between between how much water a hotel/hostel resident uses for bathing, and how much they use for toilet-flushing. This makes GWR an excellent way of reducing the mains-water consumption of establishments such as hotels and hostels
GWR are generally more costly then RWH systems, but have the characteristic of vastly reducing the mains-water consumption of, say, hotels and hostels where bathing and toilet-flushing account for the majority of their water consumption. System suppliers are able to demonstrate good payback periods for such systems, which can also play a key part in meeting organisational environmental policies, and thus desirable accreditations such as B-Corps
Although a market is beginning to develop for domestic-scale GWR systems, many/most to date in the UK have been fitted to new-build hotels and hostels in response to the growing pressures on mains-water supplies and associated reviews of organisational environmental policies.
Usually the suppliers of GWR systems will offer a "turnkey" solution, that also includes ongoing maintenance.