Attribute Definition
Attribute definitions describe the properties that "make up" each asset type. Each attribute definition describes the type of data it can contain as well as whether it is required, read only, multi-value, and many other qualities. Attribute definitions are identified by a name that is unique within its asset type. Attribute definitions are defined as either value types or relations to other assets. Further, relation attribute definitions can be either single-value or multi-value.
For example, the Estimate attribute definition on the Workitem asset type is a scalar (specifically, a Number). On the other hand, the Workitem asset type's Scope attribute definition is a single-value relation (to a Scope asset). The reverse relation, Workitems on the Scope asset type, is a multi-value relation (to Workitem assets).