-1 means use the servers stacksize setting, which could be controlled by any plugins that modify stacksizes, such as Stacksize Controller
set it to any value to use that value instead of the servers value. it is useful when you want to split items into many boxes
so, if it randomly picked one stack of 30 bandages then it would be split into 3 stacks of 10 that then would randomly spawn into boxes. this is especially useful for boom items.
each split item is counted towards the total number of items required to spawn. 1 inventory slot = 1 item spawned
stacksizes for resources are ignored in the TC and use the servers stacksize instead.