Simplex is a means to communicate between radios without a repeater
Simplex communicates like a walkie-talkie using line of sight transmission without a repeater. This is important in an emergency in case no repeaters are available. We periodically test our ability to reach far away stations using simplex transmissions. Messages can be relayed from station to station to eventually arrive at the intended recipient.
San Mateo County conducts simplex drills and so does SC4ARC. After the test, a matrix can be drawn up showing who can communicate with whom.