Although a year later, I stumbled across this and then found some guidelines which I am adapting to my needs, so thought I might share my findings.
The ISO/IEC 81346 one letter codes are available online. There is also a introductory video explaining the Reference Designator System they work on for the "common language" attempt for systems. Here is a compiled/merged list of the one Letter codes from their current edition 2016 and 2017 draft.
- A: (multi_function) Two or more purposes or tasks
- B: (sense) picking up information and providing a representation
- C: (store) storing for subsequent retrieval
- E: (emit) emitting
- F: (protect) protecting against the effects of dangerous or undesirable conditions
- G: (generate) providing a controllable flow
- H: (process matter) treating matter
- K: (process information) treating input signals and providing an appropriate output
- M: (drive) providing mechanical movement or force
- N: (cover) enclosing partly or fully another object
- P: (present) providing perceptible information
- Q: (control) controlling access or flow
- R: (restrict) restricting or stabilising
- S: (interact) detecting a manual action and providing an appropriate response
- T: (transform) transforming
- U: (hold) structural positioning of other objects
- W: (guide) leading from one place to another
- X: (interface) interfacing an object
Taking as reference examples Li-aung Yips' answer, I would interpret to use the following codes.
- Contactor : Q
- Relay : K
- Switching devices for control
- Switching devices for power : Q
- Solenoid : QM ?
- Terminal : X