The output of a Filter Step depends on its operator:
- Most operators derive their output from their input. For example, many of them output a subset of their input, and thus truly live up to the name of "filters", narrowing down the overall output of the containing run. These operators are called selection modifiers.
- A few operators ignore their input and generate an independent output instead. These are called selection constructors: they construct an entirely new selection.
A good example of a constructor is
title. The output of
[title[A]title[B]] is just
B. But the
field operator is a modifier, so
[title[A]field:title[B] outputs nothing at all.