SYMFILL isn't as exciting as it sounds. It fills something with symbols (It's called "Symbols in Area" in the menus to distinguish it from the FOREST command, which is "Fill with Symbols"). SYMFILL can make orchards or random messes, scale things by distances from the edge, align things to the edge tangent or perpendicular, and do lots of stuff that involves a placing things onto a jittered grid with a distance field value/gradient computed from a set of entities. Some examples of its use appear in http://forum.profantasy.com/comments...cussionID=7861 and https://rpgmaps.profantasy.com/autom...drawing-tools/ and https://rpgmaps.profantasy.com/drawi...attered-woods/ at the ProFantasy blog. Your mentioning an animated brush (random groups on the symbol placement tools in CC3+) brought it to mind because it's basically an automated animated brush for those of me who are too lazy to click a lot.

SYMFILL is the 2D analog of the ESC (escarpment), which puts things along the edges of entities. They have similar dynamics, but each is a little better adapted for its intended purpose. The FOREST command in CC3+ is a bit different than SYMFILL in that it uses blocks of symbols of various size in order to tile an area on the map. SYMFILL has better dynamics, but FOREST can tile an area in fewer symbols with the right set. Tiling with as few symbols as possible was very important in CC2, but CC3+ is a bit more efficient.