When people decide to start their own training seminar business, even if they have been employed as trainers before, they tend to be concerned that defining narrow subjects or topics to offer training about will restrict their abilities to make a living. So, what they do is try to be training generalists, offering a large laundry list of courses and seminars that have little in common with each other.
The big problem with this is that trying to be everything to all potential customers sends the message that the trainer is an expert in nothing. Another problem is that it's unlikely that anyone can be an excellent training generalist, since it's probably impossible to be an expert trainer in dozens of diverse fields. It may be possible to be OK, but not great, and it's being great that makes a difference over the long term in the career of a training professional.
So, in short, if you are starting out, it's better to carve out a niche of a few topics, and target markets than to claim you can do anything. That's provided you are opening what is essentially a one person training business. Of course if you are partnering with other people, your niches can be larger, taking advantage of the skills and abilities of the various partners.
One of the biggest mistakes a person can make when starting a training and seminar business is to try to be everything to all. It doesn't work. It results in lower fees, and low credibility.