BFA stands for Bonus From Allies.
BFA is how much power you get from your allies' strength. Each player gets exactly 2% of their allies' combined raw stats.
To explain this in further detail, let's take a look at a picture. Below is a snapshot of my current raw stats.

As you can see, there's quite a bit going on here. It's easiest to think of this in 4 categories.
- Current Troop Strength
- Current Spy Strength
- Max Troop Strength
- Max Spy Strength
The numbers to the left of the "/" symbol are your current troop/spy strength. The numbers to the right of the "/" symbol are referred to as your "raw stats." These figures represent the maximum possible strength your kingdom can have (not including your allies, equipment, or other bonuses). This is only a measure of the strength of your buildings and castle.
So, if I take my raw troop stats and multiply them by the scaling factor of 0.02, then I'll get my Bonus To Allies (BTA). This figure is how much my owner gets for having me as his/her ally. My BTA gets added to his/her raw stats.
This is probably starting to get a bit confusing, so let's do an example.
Here is my profile as viewed by other players.

Notice that these stats are different than my raw stats. That's because other players only see my BTA. That's because BTA is the important information that my potential owners are interested in, since only my BTA will be added to their raw stats. But I digress.
So if someone hires me, they will get my BTA added to their stats. In the case of my current owner, Donno-The-First, he will get my BTA added to his raw stats.
But wait! We can't see his raw stats!
Don't worry. To find someone's raw stats, simply look at their profile and multiply their BTA by 50 to find their raw stats.
So, here's Donno's current profile:

Rememeber, we only see his BTA. So to find out his "stats," we have to add his BFA to his equpment stat bonus. For simplicity, let's ignore his equipment stats. So, if we're working with only BTA stats, he has 11,967,224 attack stats.
My ally bonus, as shown above is 912,696. But my ally bonus doesn't get added to his BTA - it gets added to his
raw stats! So you have to multiply my stats by 0.02 to find out the BFA he gets from me.
So, 912,696 x 0.02 = 18,254. Remember, this is only the BFA he gets toward his troop stats from owning me. So my contribution to his troop attack BFA is 18,254, which means if I was his only ally, his effective troop attack "stats" would be 11,985,478.
If that wasn't confusing enough, here's some more technical details to throw you for a loop.
BFA only works one level up. In other words, my owner's owner doesn't get any stat bonus from my owner's allies.
Don't be ashamed if you have to read through this multiple times for it all to sink in. It took us all a while to really figure it all out.
