Residential memory is part of the process memory that is now swapped and is in RAM. This includes also memory shared with other processes so sum of RES for all processes may be greater that total physical memory.
Thanks very much, Glyn, Jeff, and Tom. That was very clearly explained.
A related case, see the following top dump. The Postgres process is using 87g residential memory, which I thought was the physical memory consumed by a process that can't be shared with others. While, the free+cached is about 155gb. But, (87 + 155) is bigger than the total available 198g RAM. Does this mean some of the residential memory used by Postgres is actually shareable to others?