Importacular links constituents, individual relationships and organisation relationships to the data in the Raiser's Edge via Criteria Sets (read the User Guide section about Criteria Sets here).
Changing your Criteria Sets will change what records in the Raiser's Edge are matched to the data in your data source. You can make them more restrictive or more lenient depending upon your needs.
For a list of suggested Criteria Set changes, see this knowledgebase article.