There are several possiblities -
A portal is used when, for instance, you have a Contacts table
Contacts table has a contactID key and a companyID key OR companyname field
A second instance of Contacts table is made and named Contacts2 or ContactsSelfRelate or something.
A relationship is created between Contacts::companyID and ContactsSelfRelate::companyID.
This allows a portal to be created on one or more of the Contacts Layouts that will list in the portal all Contacts that match the companyID in the currently viewed record.
Google "filemaker portal examples"
How do I create and sort portals?
The Computer Guy, Seattle
In the Estimate table there could be a portal for equipment listing however many items are needed to fulfill the Estimate. A field in the Estimate Table would total the items from the portal to the Estimate and where you could apply general miscellaneous charges such as delivery or sales taxes.