A custom rule is created from a template: it is a built-in rule for which you specify one or more parameters that affect the behavior of the rule. Compared to built-in code review rules, custom rules are more powerful and require slightly more configuration. You first select a rule template and then specify values for the template's parameters. A severity value is set and processed exactly as in a built-in rule.
A custom category is a folder for storing custom rules. A custom rule cannot be created unless there is a custom category in which to create it. A custom category is created inside a parent category, which can be either a built-in category, such as Performance or Program Structures, or another custom category.