In electronics, the notation is usually used to indicate the direction of current or voltage, while in mechanical engineering it is sometimes used to describe the force or stress between two points, and sometimes even a component that spans between two points (like a beam on a bridge or truss).
[2] This is the basis for a convention to standardize the directions of double-subscript labels.
For example, a power input labeled VCC would be a positive input that would presumably connect to the collector pin of a BJT transistor in the circuit, and likewise respectively with other subscripted letters.
The format used is the same as for notations described above, though without the connotation of VCC meaning the voltage from a collector pin to collector pin; the repetition avoids confusion as such an expression would not exist.
The table above shows only the originally denoted letters; others have found their way into use over time, such as S and D for the Source and Drain of a FET, respectively.