2.2.1.1 SecurityTokenReference Element
The <SecurityTokenReference> element is specified in [WSS1] section 7.1, [WSS] section 7.1, and [BSP] section 7.1. The <SecurityTokenReference> element MUST contain exactly one of the following elements as a child element:
A <Reference> element as specified in [WSS1] section 7.2, [WSS] section 7.2, and [BSP] section 7.2. This document refers to this element as a direct reference.
A <KeyIdentifier> element as specified in [WSS1] section 7.3, [WSS] section 7.3, and [BSP] section 7.4. This document refers to this element as a key identifier reference.