This is a train pickup (aka provider) station for generic schedules.
It automatically balances chests when loading them (you'll need to handle belt balancing of input belts yourself though). This ensures all inserters can be busy full-time while loading a train, thus reducing load times.
Additionally it calculates the amount of trains it can fill up with the provided resource. It then looks at the green circuit network for requested resources and sets its train limit to the smallest (min) of the request count and its own availability count.
Requests on the circuit network should be the negative number of trains.
E.g. -7 coal means that in total 7 trains of coal are requested somewhere. If at that time it only has 3 trains worth of coal stored it'll set it's train limit to 3. If however it has 12 trains worth of coal it'll set it's limit to 7. (I.e. it's limit will never exceed the amount requested nor the amount available.)