Minecraft wiki entry for Unused_textures, associated text: Since this was added in the same version that implemented new flowing mechanics for water and lava, it is extremely likely that this was used to test its flow directions: the presence of two sets of icons implies one set was used for water and the other for lava (further reinforced by one being blue and the other pink), the circle would represent still fluids, the eight orthogonal/diagonal arrows would correspond to orthogonal flowing, and the downwards arrow the sides of liquid ("waterfalls").