A textual filter is simply a representation of a filter clause in textual format.
You can use textual filters as arguments for various commands. For example, the Find command can be used to search beneath the current folder for files greater than 1mb in size:
Find HERE FILTERDEF size match > 1 mb
The FILTERDEF argument in the above example introduces the textual filter - everything following it is the filter definition.
The easiest way to create a textual filter is to use the filter control to define the filter using the user interface, and then click the Edit as text button. This will convert the UI-based filter into a text based one. You can then copy the text out to use on the command line.
You can also build a textual filter from scratch using the various keywords.
Various commands accept a textual filter as an argument - Find , SetAttr , Print , Copy and Delete all have a FILTERDEF argument that lets you provide a filter. The FILTERDEF argument is always defined as a /R "raw" argument - meaning that it must be the last argument given on the command line.
You can use the multiline command syntax in button definitions to provide a filter definition that spans more than one line. For example,
Find IN C:\ [[ FILTERDEF size match > 100 kb and size match < 200 kb and name match *.jpg ]]
If a textual filter beginning with a = character it uses the Evaluator - they aren't traditional filters.
See the Evaluator Filters and Find page for more information.