Xoom identities are assigned to items via the xoom:id attribute; references are indicated via the xoom:ref attribute.
Here is an example of a Rule, called _Pre-SOT - Match
Skills, as it appears in XoomXML.
As an item, the Rule is given its own, unique, environment-independent Xoom identity via the xoom:id attribute, highlighted in blue. Its full Xoom identity is: Rule[_Pre-SOT - Match Skills]. References are indicated via the xoom:ref attribute, which takes the value of the xoom:id to which it points. This particular Rule matches engineers' skills with those required by the task and so references Engineer Property[Skills] and Task Property[Required Skills] (highlighted in red) amongst others.
