If the court is being asked to determine any defendant's rights or obligations, it must have the power to make orders concerning the individual defendant. This is called personal jurisdiction. Personal jurisdiction is also called "in personam jurisdiction."
For a court to have personal jurisdiction over a defendant, the defendant must have been personally served (or have accepted service of the court papers) and the defendant must have at least some contacts with the state in which the court is located. No set number qualifies as the minimum; each situation must be analyzed case by case. If the defendant lives out of state, the court must look at the defendant's contacts with the state. Going into a state regularly to conduct business is usually sufficient for the court to obtain jurisdiction; sending child support payments to a state, without actually visiting the state, however, is not.
Example: Denise and Walter spent their entire married life in Colorado. Denise moved to New Mexico, established residency and sued for divorce. If Walter has virtually no contacts with New Mexico, the New Mexico court has no personal jurisdiction over him. As a practical matter, this means the court may award Denise a divorce, but cannot make any decisions affecting the division of property, an award of alimony or child support, or a determination of custody and visitation because these matters affect Walter's rights as an individual. If, however, Walter and Denise spent five weeks every summer during their marriage in New Mexico, the court may rule that Walter's contacts with New Mexico are sufficient for there to be personal jurisdiction in New Mexico.
This can be a complicated and convoluted area of law with many pitfalls and obstacles should opposing parties decide to contest personal jurisdiction.