The Java EE 6 Tutorial, Volume I

The id Attribute

The id attribute is not usually required for a component tag. It is used when another component or a server-side class must refer to the component. If you don’t include an id attribute, the JavaServer Faces implementation automatically generates a component ID. Unlike most other JavaServer Faces tag attributes, the id attribute only takes expressions using the evaluation syntax described in The immediate Attribute, which uses the ${} delimiters. For more information on expression syntax, see Value Expressions.