The list of eight criteria below probably isn't a bad start.
1. common set of easily identifiable needs, problems that you can help them with
2. common lifestyle: desires, passions, values, interests, hobbies, that you can cater to
3. established, high quality hubs, communication networks etc. You can find them and reach them easily.
4. there’s enough of them to meet your needs
5. they’re fun to work with and in alignment with your nature (these are likely the kinds of clients you most easily and naturally attract and the ones to whom you feel the most attracted - it’s a niche or community that you want to see thrive).
6. they are underserved
7. they can afford to pay you full price for your products and services
8. they are in alignment with your long-term business goals (e.g. if you do eco-lawn care and want to work with “Golf Clubs" eventually, but now need to do residential, it might be wise to focus on people who golf for now so that you can bridge into that later).