Don't assume that the notation you use will always be familiar to everyone.

A lot of the notation we use is just what was chosen (pretty much at random) by the first person to consider the problem.

Sometimes this notation becomes adopted by enough people to become a standard convention,

but this should not be assumed.

It may be that the notation you are using is particular to your tutor, or text book.

Thus in your initial post:

What is the definition of di, do, Peye ?

As an aside have a quick glance at this Wikipedia article <

Wikipedia : Light Speed Notation>

Here it explains that even (arguably) the most famous symbol "c" for the speed of light is not completely universally uniquely defined.