Categories list profiles that share some common thread, for example all the people buried in one cemetery are a category, all the passengers of the Titanic are a category.
While most people could be placed in thousands of different categories, it is usually best to put them in categories that potentially help further our research, ones that help us connect individuals, or ones that can add context to the persons life. For example, multiple children of one family may be baptized at the same church so a category for that church may help us connect individuals. The journals of a soldier may contain mentions of the comrades he served alongside but even if they don't the experiences of that soldier may add more context to the life of someone who shared their experiences - adding them to a category such as their military unit allows us to more easily 'browse' these other possibly non-related people looking for clues.
