There are two primary ways of structuring your seminar and training offerings, but unfortunately there's no standard way to refer to them that is understood by everyone. They are important because the process of marketing and designing training is different for each one, so it's something that should be considered when setting up your training business.
Training for intact work groups works this way. You have a client, or contact person from a specific organization who hires you to deliver training to some or all of the employees in that specific organization. It might be training one group, or a number of groups. People from outside the organization cannot attend, the organization handles the registrations and nuts and bolts (usually), and the organization may expect you to customize your course to suit the specific target audience in the organization.
That's different from an open seminar, where individuals sign up on their own to attend, and you may get people from many different organizations attending. For example, when you see an advertisement for a seminar in the newspaper, that's an open seminar. You could register. Your neighbour could register. Anyone could register.
There's no reason why you can't do both, and there are advantages to doing so, since participants who attend open seminars tend to sometimes want to hire you to work with intact work groups. However, the skills and time involved to do open versus intact work group training are quite different. If you put on open seminars on your own, you need to market, register, do all the materials, book rooms, refreshments, etc, and you shoulder any risk if people don't attend or pay.
There is a middle ground and that is that a trainer can work for another larger company that specializes in sponsoring and organizing training events and courses, but in effect you are workikng as a quasi-employee.