A Form C relay output is a single-pole double-throw, or SPDT, relay that breaks the connection with one throw before making contact with the other, a process known as "break before make." Relays are classified into forms, the most common of which are A, B and C.
A relay can be made of "n" poles and "m" throws. A single-pole single-throw relay is an SPST. An SPDT has one pole and two throws. A double-pole double-throw has two poles, each with two simultaneously controlled throws. A SPDT relay, as with Form C, has five terminals total, including two for the coil.
Relay forms are classified based on number of poles and throws. A Form C relay output is also known as a change-over, double-throw contact, or a transfer contact ("break before make"). This type of contact controls two circuits: one normally open and one normally closed. These contacts have a common terminal. A Form C isolated relay would have three terminals.
Form C is a combination of Form A and B. It is useful for applications that need one circuit to remain open. A piece of equipment that runs continually would need a Form C relay; when one circuit is shut off, another is turned on.