Assigns one or more filters (comma separated) to be automatically applied to a variable.
As an example, multiple form variable submissions on the same page come back null-separated, like 'value1\0value2\0value3'. To automatically change those nulls to spaces, you could use this directive:
Filter mail_list null_to_space
Of course you could just as easily use the [filter] tag on the page if the filter is only going to be used in a few places. See the [filter] tag documentation (wiki FilterTag) for more information and a list of filters.