The OnAcquireItem event is fired when a creature has an item added to its inventory.
In addition to items acquired during gameplay, this includes items that are thought to be possessed when the creature is created, as the game initially creates creatures with no items (then automatically adds the appropriate items). For non-player characters, this means an OnAcquiredItem event is fired for each item placed in its inventory via the Toolset, and these events fire before the OnSpawn event. For player characters, this means an OnAcquiredItem event is fired for each item possessed by the character before entering the module, and these events fire before the OnClientEnter event starts.