This can be done a number of different ways. The most common method uses the category field as the match field in a relationship.
See "Adventures in FileMaking #1 - Conditional Value Lists" this is a demo file with 10 different types of conditional value lists--each with detailed documentation on how they work and how they were set up. The basic conditional value list example found in it should be something that will do exactly what you describe.
Thanks a lot Phil,
I'll check it !