You can use the Filter XPath property on a Repeater to select only those instances that meet the criteria in a filter expression. Filtering is performed after sorting which means that instances are selected based on sort result.
You will typically use the Filter XPath property to filter the result from a sorting criteria, but it can also be used to filter the result from an unsorted Repeater. However in that case, processing is likely to be faster if you include the filter in the XPath expression directly.