Rules builds upon on its own API, which allows contributed modules to provide

• default rules, fixed or customizable
• further rule sets, fixed or customizable
• actions, which may be configurable
• conditions, which may also be configurable
• new events, for which conditions and actions can be configured
• new input evaluators (like token and PHP input evaluators)
• new rules data types
• user interfaces

Furthermore the API supports

• multiple arguments, for actions as well as for conditions
• configuration of actions/conditions for every event and rule set, where the necessary arguments are available. Action writers don't have to care about action compatibility.
• dynamic loading of needed arguments
• condition evaluation which supports logical operations like OR and AND
• negating conditions or logical operations
• ordered execution of conditions, actions and rules, controlled with weights
• a flexible input evaluation system (token, php, ..)
• packing and unpacking arguments, used e.g. for scheduling rule set execution
• building simple/complex interfaces, like rules admin ui or transformations