The Difficult Editor is a very powerful tool for making changes to the player experience depending on the difficulty they choose. You can easily increase AI HP or acuity, give the player more or less HP, or affect how often the AI relight lights.
In DR, select MAP --> DIFFICULTY
Select the tab for the appropriate Difficulty Level. For example, let's say you want to increase AI Hit Points at the "Difficult" difficulty level. Select the "Difficult" tab.
On the right, click the "Classname" dropdown menu. This will give you a list of all the entities in TDM that you can modify. The default AI HP are set "in atdm:ai_humanoid".
Type in the spawnarg you want to change. For Hit Points, it is called "Health" (don't include quotations).
In the Argument field, you can type either an absolute value (default), like 150. That would mean all AI would now have 150 HP if the player selects the "Difficult" level. If you click the dropdown menu, you can also Add to or Multiply the current value. (Select "Multiply" and type in 2 to double the AI's HP).
The difficulty editor allows to specifically edit all spawnargs of all entities for certain difficulty levels, both setting absolute values, deleting values and changing them relative to their default value.
Examples: - Setting "is_mantleabe" spawnarg to "0" on the fence entity on Hard difficulty and above. - Increasing visual AI acuity by 60% on highest difficulty. - Decreasing player health to "30" on Hard. - Decreasing the amount of arrows the player gets in the armory, by setting the "inv_ammo_count" spawnarg. - Changing an entity's "name" on easy difficulty. - etc.
Almost everything in D3 is handled by spawnargs, and the difficulty editor allows for precise control over every single spawnarg available. The settings are applied before each entity is spawned, imo it's an extremely powerful tool. The only thing it doesn't affect is entityDefs which are not actually spawned, like vocal sets.