this is all speculative, I don't think they have shown exactly how this mechanic works yet. or at least I've not seen it. but If you have units out there with a mix of panzer III and panzer IV and you order up a new panzer division they are gonna get all panzer III's - because if you had any of those panzer IV's in your stockpile, they'd of been shipped out to the existing divisions and panzer III's put back in the stockpile; so your stockpile in that situation is nothing but panzer III thus the new division full of panzer III's jumps right up to the top of the Queue for new Panzer IV's cause it has none, and as tanks roll off the lines it soon too will become a mix of those units. I think this is how it will work, but again I have not seen it.