On any typical non-pressure, man-to-man defense, it’s a simple math proposition: there are five eligible pass catchers on the field and seven pass defenders (assuming a typical four-man rush), meaning you have two “help” chess pieces available. You can use the two extra defenders as zone players in the deep halves of the secondary; everyone else plays man-to-man with no low help. Another option is using one deep, middle-of-the-field safety, affording the defense one underneath coverage “help” zone player (the “rat”). Alternatively, the help player can be used to specifically double one receiver, while still keeping a deep middle of the field safety — but then the zone helper is gone.