Well. It is a mule, and it's currently "muleing." .getFreeCapacity(resource); is documented. For non general stores, you need to specify what you want the free capacity of. The reason for this is simple. What if you have something with a store that accepts only a subset of a few items? Like a lab? Or a nuker? Labs can hold 5K TOTAL, but 3K of it must be that lab's mineral resource, and 2K of it must be energy. You have to tell it which you want the free capacity of for it to be meaningful at all.