Here is a good explanation of ectopic rhythm from HealthLine https://www.healthline.com/health/pregnancy/ectopic-heartbeat
The article explains:
"Premature atrial contraction
An early heartbeat that originates in the heart’s upper chambers (atria) is a premature atrial contraction (PAC). In healthy children, irregular heartbeats are almost always PACs and are harmless.

Premature ventricular contraction
When the irregularity comes from the lower chambers of the heart (ventricles), it is called a premature ventricular contraction (PVC). The risk of PVC rises with age. You are at increased risk of PVC if you have a family history of PVC or if you have had a heart attack."

@lolly906 I've attached a diagram of the heart showing the location of the atria and ventricles. Sometimes seeing an image helps to recall which is which.