CRM Alternate Key:
Is an optional, unique ID value you define in Dynamics CRM.
Any decimal, integer, or string field can be used to define either a simple alternate key, defined by a single field, or a complex alternate key, defined by multiple fields.
An alternate key has both a display name that appears in the interface, and a key name that Dynamics CRM uses to identify the field.
By default, the key name is the display name with a new_ prefix. For example, if you define the display name as MyKey, the key name assigned is new_MyKey.
For the Upsert operation, TIBCO Scribe has created a virtual field named vfAlternateKeyName.
This field needs to be populated in the DTS with the name of the CRM Alternate Key field.
See the attached document for more information