The UT807B is sized to perfectly sit atop tail bags like the UT806 or side cases like the Trekker Outback or Dolomiti. It can however function as a stand alone tail bag as well. A waterproof inner liner keeps contents dry.
The Givi GRT702 is a roll-top dry bag with integrated loops to support stacking from other bags within the Gravel-T series. A clever air purge valve makes closure a cinch since you're not fighting any trapped air build up as you form the water-tight seal.