I finally think I figured out what this stat does!!! /does a jig. I recruited a spy in London and he turned out to be a crappy one, age 38 (feeling his age) and only +1 natural spy skill. Since he is feeling his age he also has the stat "+1 training agents".

I originally thought this had something to do with creating new spies. I was wrong. After leaving this spy alive for almost 20 turns this guy just keeps cranking out spy ancillaries. I've gotten 4 so far, beguiling bard x2, pickpocket x2 (I made most spy ancillaries transferable). I could be wrong and those ancillaries are being created due to me having a spy in my capital which is well progressed but then again...

What do you think?

Morty