These bots are summoned by the add_bot() Jabber-RPC request, made from a player to a referee.
It's up to the referee to know what bots it offers, and when it's legal to add them to its table -- it could be limited by table space, computing resources, and so on. If a ref that offers more than one bot-type receives an add_bot request when table-space is available, then it initiates the bot choosing procedure, sending a Jabber data form to the client, and creating a new bot based on the client's response.
Throughout Volity documentation, references to "bots" usually mean retainers. Ronin programming is an interesting exercise, and is an exercise left to programmers. :)