It is more about what each symbol means.
If I select only water (right?) I cannot add fish, but if I select bump (left) and waves(right) I can add fish.
If the half circle (middle) and waves (right) I can add things to the water.
What are the permutations of the three symbols.
Maybe a video will help to understand the use cases.
https://gfycat.com/sprybriefamurstarfish
It basically allows you to choose where things get painted. In the example above, i only want flowers on the land and ferns on the rocks
water does not let me drop things into the water. I must also select one of the other icons.,