Split on hit: If a weapon damages an enemy, but does not kill it, then the enemy will divide into
n copies of the npc that you specify in the attributes field.
Split on Death: When the npc's HP is <= 0, it will split into
n copies of the npc that you specify in its attributes.
There is not yet an enemy editor weapon defence that causes npcs to split based on weapon types, although this is planned. It's purely HP-based for the above two.
Once there is a working split defence, Digdogger will be using it.
To split npcs in custom ways, at present, does require scripting.
Note: Splitting npcs can be set to split into more copies of themself on hit, or on death. If you give an npc the split on hit attribute, and you make weapons that OHKO it, you can create hellish abominations, such as my wand ghosts, and arrow knights.