Along with the standard character equipment and armament that can be assigned to characters, catalogs can also contain rules that apply to your game. These can be quick references to official rules provided by Chaosium or home brewed rules that are not covered by official rules. These might be things like "Surviving the vacuum of space" or "Level 1 Trauma Hospital HP Recover Rate". Many times you may want a place to store rules that are going to be relevant in the games you run.
Rules are categorized (these are customizable in the Select Lists Editor area off of the Main Menu) and can have their source noted (Official, Official/Altered or Created) and any image assigned if one is needed. As with other areas of the system, the P button over the image field lets you send the rule image to the Players Window if it's open and accepting input.
When you create rule entries in a catalog, if that catalog is linked to a package or scenario, the rules become available to be linked to scenario parts. This gives you quick access to review the rule(s) when you are running your game and have the current scenario part open.