An Integration ID is automatically generated as the unique identifier between the two records when they are linked via Integration between The Raiser's Edge and The Education Edge.
From the Relationships tab of a record, you can right click the relation and select Properties. In the Individual Properties screen there is an Individual Integration ID, which is the the relationship's Integration ID. For example from the Relationship tab of John Smith's record, highlight his son Jim's relationship record and select Properties.
The Individual Integration ID listed is essentially Jim's Integration ID that matches the Integration ID in EE, found by viewing the Properties of Jim's Education Edge record.