Difficulties - Difficult and Impossible.

Don't overcomplicate your question though. :) It’s quite clear what you mean:

Can you tell me what the difference is between “difficult” and “impossible” when creating a map?...
I can’t find the difference between *difficult* and *impossible*.

You just liked Lider1’s answer. :)
It’s much easier to create strategic maps (in the style of random or standard maps) than RPG ones.
If you want the AI to exhibit specific behavior in your map, it’s not that simple. Of course, you can adjust its power and development speed with resources and troops, but to make the enemy go exactly where you need them to and do exactly what you need them to, you have to create conditions and constantly test the gameplay. This is easier on a map with closed areas; if the areas are open, everything might not go as planned. This is an entire science that the authors of such maps could teach. I can only speak as a player and theorist. Recently, I made some minor edits to the "War of titans" map for my own purposes, which I wrote about here.
The AI can sometimes behave surprisingly, and even differently during two playthroughs of the same map. For example, on the Polish map “Zagubione Imperium,” the last enemy, Red, stayed put and didn't even leave his city until I came and killed him. But another player had a very active Red, and victory required effort.