I have a question.

[C0sm0s-Chill]

Hello , is it possible to sperare the id of the npc of the negative ? to be able to configure loot to each npc
Thanks ! :)

have a good day.

[Arthri]

It depends on which you are talking about. Generally speaking, yes, the netID of an NPC can be negative. https://terraria.fandom.com/wiki/NPC_IDs. However, those negative IDs are simply translated to positive npc types with certain properties a bit different. NPC types can't be negative.

[C0sm0s-Chill]

Let's say I use the
LootTableEditor plugin and when I put a loot on the blue slime which has the Id "1" All the other slimes have the same loot .

[Arthri]

I don't know how LTE handles it but I believe no. You have do it manually for each mob