In partial fraction decomposition, the degree of the numerator must be less than that of the denominator by at least one to ensure proper splitting of the expression. If the numerator's degree is equal to or greater than the denominator's, polynomial division is necessary to simplify the expression. This results in a polynomial part and a proper fraction where the numerator has a lower degree than the denominator. The numerator can have any degree up to one less than that of the denominator, and it's possible for the leading term's coefficient to be zero. Understanding this relationship is crucial for correctly applying partial fraction decomposition.