Machine Sites are locations in Horizon Zero Dawn where certain types of machines regularly spawn in groups. Machines sites appear on the map as the symbol for whatever type of machine spawns there. All machine sites, except for some Shell-Walker and Behemoth sites which can be mobile convoys, are stationary.
Machine sites are not part of any quest. Rather, they are simply ways of letting the player know where certain types of machines are guaranteed to appear in varying numbers.