Why Satellite mode designations list the uplink band first and the downlink band second, using letter codes for the bands: U means UHF 70 centimeters (around 435 MHz) and V means VHF 2 meters (around 145 MHz). So U/V means you transmit up on 70 cm and listen down on 2 meters. This is the most common linear transponder arrangement on modern amateur satellites, including the ISS-style and AMSAT birds; the reverse, V/U, uses a 2 meter uplink and 70 cm downlink.
Watch out The 15 meter up, 10 meter down pairing describes the older HF mode A (sometimes shown as H/H letters), not U/V, and neither letter has anything to do with ultraviolet light or variable frequencies.
Up first, down second. U = UHF 70 cm, V = VHF 2 m. U/V: talk up on 440, listen down on 145.
U is for Up and for UHF. U/V: your Uplink goes Up on UHF (70 cm), the downlink comes down on VHF (2 m).