Questions:

A source charge generates an electric field to exist in the region of space around that source charge. The electric field exerts a force on some test charge that finds itself in the region of space where the electric field exits. The source charge or else distribution of source charge are the cause of the electric field. The test charge is the victim of an electric field.

In the circumstances of the electric potential when we write

U = qV

Is the q the source charge or else is it the test charge?

Answer:

We have utilized some jargon here. By a "source charge" we signify a particle that has an amount of charge equal to what we call the source charge. By a test charge we denote a particle that has an amount of charge equal to what we call the test charge. Source charge affects an electric field that with which we can associate an electric potential V. Therefore source charge causes V. A test charge q which is a particle with charge q that finds itself in the electric field will have a potential energy U. That potential energy U relies on the electric potential V caused by the source charge and on the charge q of the test charge as

U = qV