Officially, the mugs are only for the resort you bought them at during the stay when you bought them; however, many folks have reported here that they use their resort mugs at return visits to the resorts
I recall at least one story where a CM at PO-Riverside said they like to see this, as it means the resort has developed a loyal following.
Now, as for using the mugs at a different resort than you bought them at, it appears to be more likely that the official policy will be enforced and a CM may tell you that you can't refill the mugs.