Double subscript notation
In engineering, double-subscript notation is notation used to indicate some variable between two points (each point being represented by one of the subscripts). 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). Note that, although there are many cases where multiple subscripts are used, they are not necessarily called double subscript notation specifically.[1]
This is a convention to standardize the directions of double-subscript labels. The following uses transistors as an example, but shows how the direction is read generally. The convention works like this:
means the "V" from C to B. In this case, V would probably stand for voltage, C would probably denote the collector end of a transistor, and B would denote the base end of the same transistor. This is the same as saying "the voltage drop from C to B". Although this assumes what the letters are, presumably this notation can be used in other contexts.
means the "I" from C to E. In this case, I would probably stand for current, C would again probably denote the collector end of a transistor, and E would denote the emitter end of the transistor. This is the same as saying "the current in the direction going from C to E". Although this assumes what the letters mean, presumably this notation can be used in other contexts.
The names given to power supply pins on integrated circuits are independent from the conventions used in double subscript notations. They are coincidentally named in a similar way, for example VCC.