Angel: How do crawler and faster logistic bots negotiate for work?

[wheezil]

I am playing angel+bob mod pack and recently researched "normal" bots. I've noticed that I can mix both logi bot types in the mesh, and they seem to cooperate. But given the vastly different speed/capacity profiles of the two technologies, how do they decide when a slow/big bot should be used vs a fast/small bot? I've started using personal logistics (which I avoided before because the crawlers are so slow), and it seems like the fast bots kick in if something doesn't happen fast enough, but perhaps I am imagining that?
Thanks
john