How Does Safe Enchantable Level Exactly Work?
If an item has a Safe Enchantable Level of +3, does it mean that one can enchant it one more time when the enchantment is at +3, or does it mean that one needs to stop at +3?
It's the level where you can enchant the item one more time safely.
However, even if you enchant an item above its safe enchantable limit, it may not be destroyed. For more details, see Item Evaporation Chances.