what is UC input for a relay?
Standard a relay is designed for VAC or VDC input only. In the case that the relay input is designed for VUC both kinds of supply are allowed.  In other words. It is safe to supply the relay with VDC or VAC. The letter U stands for universal. A little rectifier is build in the relay to make this possible. The danger of burned relays because of using an VAC relay when a VDC relay is required is gone that way.