Drafting geek alert. Training Volume is based upon time spent training, and frequency of training. In a sense, it creates a 2-axis coordinate system that can be uses to show “volume”.
I see time as the vertical axis (hours as units) and frequency as the horizontal (days as units). You can create a line graph between the time spent training on each day. When you do, you effectively create and “area” of training under that line. I could see calling this area “volume” and is a decent visual representation of the amount of training.