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.