A more thorough explanation:

The main-relief rule is a legal principle that allows a plaintiff to choose the venue for a lawsuit based on the primary relief they are seeking, even if other claims included in the suit would not support venue on their own.

Let's say a person is injured in a car accident in California, but the driver who caused the accident lives in New York. The injured person could file a lawsuit in California, even though the defendant does not live there, because the primary relief sought is compensation for the injuries sustained in the accident. The fact that the defendant lives in New York would not prevent the lawsuit from being filed in California under the main-relief rule.

Another example could be a case where a plaintiff is seeking a restraining order against a defendant who lives in a different state. The plaintiff could file the lawsuit in their home state, even though the defendant does not live there, because the primary relief sought is the restraining order.

These examples illustrate how the main-relief rule allows plaintiffs to choose the venue for their lawsuit based on the primary relief they are seeking, rather than being limited by the location of the defendant or other factors.