I believe they relate to what the factory is building at the time. So in that context 'BP' is relating to the current build percentage of the unit it is making. If a bot factory is building a dox and it's 'BP' is 50% then the dox is half finished. The 'eff' is referring to the current build efficiency. If the factory has full metal and energy then this should be 100%. If your metal and energy is low, then the efficiency will drop. A build efficiency of say 50% means that the factory is only running at half speed (i.e. it will take twice as long for it to produce each unit). At 100% it's running flat out so you can't produce any quicker with that factory (and I'd suggest you probably need to make another factory as your probably wasting resources at that point).
What I mean is what BP and EFF means when you hover over a factory in a fabber's menu, and the tooltips that pops up tells you BP:number and EFF:number
building percentage means the degree of completion of construction of said unit or structure and .... efficiency is somewhat of a new notion introduced by a modder (who's mod got it's own take done by UBER) thaty tells you how hard (or not you are stalling) because of insufficient funds to construct what you asked. If you are at 100% you are not stalling and are building at 100% speed. if you are at 50% you are building half as fast as you could be. seen like this it could seem that is a high risk to be stalling but because balance is crud it's actually a-OK to build on 4% efficiency. /S EDIT : dammit ninja. it means that that factory is THIS done building it's unit and is building with THIS efficiency.
Actually if you're running terrible efficiency due to energy deficient you do build MUCH slower. On the flip side it's not as hurtful to go metal negative... Which I'm feeling should be flipped. (As obviously metal should be the llimiting factor, not energy)
BP: Build Power. The metal usage/investment rate. High build power will have a larger impact on your economy, but build things faster. Eff: The ratio between metal and energy usage. Since it's small, it must be metal/energy; so a basic fabber is 10/1000 = 0.01. I think I might have preferred energy per metal, because integers. Percentages would also get you an effective *100. As is, higher is strictly better.