Level 2 allows reserved string expansion (<a href="http://tools.ietf.org/html/rfc6570#section-3.2.3">Section 3.2.3</a>)
Level 3 allows reserved expansion with multiple variables (<a href="http://tools.ietf.org/html/rfc6570#section-3.2.3">Section 3.2.3</a>)
- Companion
- object