Categories in Tournamenteer are used to divide a single event into multiple distinct competitions. As an example, if you were running a tennis tournament, you might have men’s singles, women’s singles and mixed doubles categories. Or if you were running a schools’ football tournament you might have one category for each age group.
The key thing about categories is that, although they are part of the same tournament, they are completely separate from each other with different titles/prizes at stake in each. Teams or individuals in category A will not compete against those from category B at any point.
If you think that you need some cross-over between categories, reconsider whether you can achieve what you want by changing the tournament structure within a single category. If you need help on how to achieve this, post a question on this forum describing your scenario.
For many tournaments you will only ever need a single category with everybody competing against each other, so this is the default in Tournamenteer, but you can easily add more if required. The maximum number of categories you can have in one tournament depends on your subscription plan. If you need more you can change your plan at any time.