Decals in Visualize are like digital stickers, and only planar projection. These digital stickers wrap perfectly over the surfaces below, and you can also add any Appearance to the Decal, as well as any texture mapping...which can create some awesome effects! Just drag/drop and place the Decal on your part, then drag/drop the Appearance onto of the Decal.
For anything complex that needs to wrap over a very curved surface, such as around a water bottle, or shirt sleeve, texture mapping is required. Please watch this old Bunkspeed tutorial below, describing the differences between Decals and texture mapping, and when to use each.