220.127.116.11.2.2 Property Value-Based Explicit Tables
When a specific object in the address book and a property on that object is specified to the server via the NspiGetMatches method, the server reads the values of that property and constructs a list of Minimal Entry IDs based on a mapping between the values and other objects in the address book. This is not possible on all properties, only on those properties for which the server can establish a reference between the value of the property and some object in the address book. The Exchange Server NSPI Protocol does not constrain how a server establishes this reference. Clients can identify the properties that the server can map by trying to obtain such a table. The server MUST return an error when it cannot make such a mapping, as specified in section 18.104.22.168.10.