Found under Filters\Special
The T-Filter allows you to process the same input in a number of ways.
Let's say you wanted to first count the number of blank lines in your text and save this value to a new file/clipboard or capture variable, and then sort the text removing duplicate lines.
You would add a T-Filter, and as a subfilter of it, you would place the filters to identify blank lines (e.g. a restrict to matching lines with pattern text of '^$'), a count lines filter and the appropriate output filters such as a Secondary Output filter.
Once this side of the T has finished processing, the data is discarded, and the original text continues processing just as though the T-Filter did not exist.
Surely '^$' no longer matches a blank line!
cf. Using ^$ in a replace filter and doing a trial run does not change anything.