Electrical schematics component ID's
These are from AS 3702, Item Designation in Electrotechnology. This is equivalent to IEC 60750.

There is a three-page long table of "Item" vs. "Letter Code" including -

Contactor                        K
Relay                            K
Switching devices for control    S
Switching devices for power      Q
Solenoid                         Y
Terminal                         X




For countries following American practice, the Reference Designators come from IEEE 315-1975 and IEEE 200-1795. These are different to the AS / IEC letter codes.