Filtering records in portals
By filtering portal records, you can display different sets of records in a portal.
To filter records in a portal:
The Specify Calculation dialog box appears.
For example, if you are in an Invoices layout, in a portal that shows product records from a LineItems table and want to display just the products with quantities greater than or equal to one, use the formula If (LineItems::Quantity < 1; 0; 1).
Important  The results of summary fields, calculations, and find requests are based on the full set of related records, not just the records in a portal that are filtered. For example, if a portal is displaying a filtered subset of records, and there is a Total of summary field outside the portal summarizing these records, the summary field will total all related records, not just the displayed records.
If filtering is enabled, in Layout mode you see Filter in the lower left corner of the portal.
Related topics 
Creating portals to display related records