First of all, love the mod.
I'm not sure why, but when I place steel chests anywhere it spawns a combinator with IN and T signals. The combinator on top of the Buffer warehouse is also visible.

From what I understand in the mod code below (mod version 0.3.0), they are signals used mostly internally for identifying the warehouses

    combi.get_control_behavior().set_signal(1, {signal = {type = 'virtual', name = 'signal-I'}, count = chest_next})
    combi.get_control_behavior().set_signal(2, {signal = {type = 'virtual', name = 'signal-T'}, count = #global.chests})
    combi.get_control_behavior().set_signal(3, {signal = {type = 'virtual', name = 'signal-N'}, count = #combis})

I is the ID of the warehouse
T is the number of total warehouse (ie. the highest ID)
N the number of hidden combinators (apparently exactly 1)

Of course, if you link warehouses together, the numbers will add up.

You may want to use N in a cluster of connected warehouses to get the total capacity of the cluster (times the capacity of one warehouse, which depends on the stack size of what you store inside)

@Altaric, am I right?