Taxonomy of a Rule
Throughout the Caster documentation, you will see references to rule, command, spec, action, extra, and defaults. These all refer to parts of a Rule, typically a MappingRule or MergeRule. Both are structured the same way, as depicted below.

spec: This is what you say in order to invoke anaction.action: This is what happens when you speak aspec. Examples include Dragonfly's Key, Text, and Function actions, or Caster's R, ContextSeeker, or AsynchronousAction actions.command: The combination of aspecand anaction. In MappingRules or MergeRules, these are key/value pairs.extra: A sub-component of aspec. Examples include Dragonfly's Dictation, IntegerRef, and Choice, or Caster's Boolean.default: A default value for anextra. Optionalextras in aspecshould have default values.rule: Any of the classes which extend Dragonfly's Rule class, but most commonly, MappingRule and MergeRule.