Sunday morning talks have predefined titles and outlines. They are grouped in a catalogue which is available to the overseer and to each body of elders. Furthermore, brothers become "qualified" to give a speech on a given theme. This means that once they become auxiliary servants or elders and if they wish to give Sunday public talks, they pick from the list at least one theme they wish to represent before the congregations. Example: Brother X picks the themes "United in worship" and "How to be a good husband". He must then write his speech and rehearse it, following the Society directions, and also respect the time duration of the talk. I am uncertain if he must perform before the CO for the first talk, but I think the answer is "yes". I asume he will not be contacted by other congregations without the overseer's approval. Then once he gets the ok, he can be summoned by the congregations to give a Sunday public talk on one of the themes he's approved for. Each body of elders has a catalogue with the public speech themes and with the names and contact data of every qualified and approved brother who can give each specific talk. From what I know, only auxiliary servants and elders and maybe regular pioneers are approved to deliver Sunday morning public talks. Other rank and file brothers can give those talks only under emergency conditions, if no "qualified" brother is available.
If someone knows something else, please correct me.