How hard is it to grasp that PRCG are not techical terms? They are not scientifically derived. They are simply common lables to represent complex concepts, not unlike x, y, and z for a 3 dimensional axis. Did you need to know anything more about x, other than it's the first horizontal axis? y is the 2nd, and z is the vertical. Technical, but simple.
PRCG is the same way. Simple divisions to represent samples of data used later in a more complex way.
I'll use some big words so that Dan's simple explanation seems more technical.
Project # is the numerical designation for the initial set of work unit parameters. Project numbers are not repeated (although they have been in the past, by mistake.) They are also numerically grouped by researcher / field of study.
Run is a numeric designation for groupings of Clones with identical work unit attributes, the exact same atoms, the exact atom positions, the same temperature, except for different (randomized) starting velocities.
Clone is a numeric designation for each trajectory, and "trajectory" is the very technical thing here. See the first paragraph of Dan's FAH Wiki article*.
Generation is a numeric designation for a pre-determined length of time along a trajectory (Clone) for a specific project.
*For reference, Dan wrote, "the key idea is that of a "trajectory." You might recall Newton's Second Law, F = ma, which means that the acceleration a (change in velocity) that a particle experiences is proportional (by its mass m) to the force F it experiences. This means that if we can catalog all the forces on a particle, we can determine its acceleration. If we know the acceleration, then we can use calculus to determine the particle's position as a function of time, for all time. The result is what's called a 'trajectory' -- a kind of map of where the particle has been and where it will be going."