From the 1K Only Web Site:
On February 10, 2005, we will institute a new checked baggage policy for customers flying from or connecting from Los Angeles (LAX) to San Salvador, El Salvador (SAL), Guatemala City, Guatemala (GUA), or San José, Costa Rica (SJO).
United Economy® customers will be allowed a total of three checked bags: one free checked bag; an additional checked bag for a $100 charge; and a third checked bag for an extra $250 charge. During embargo periods, United Economy customers will be allowed only two checked bags—one free of charge and an additional bag for a $250 charge.
However, as a 1K member, you will be allowed a total of four checked bags—three free of charge and one additional bag for a $100 charge.
It's just one more way to thank you and recognize your loyalty.