A filter step represents a single operation within a filter run.

In programming terms, it is akin to a function call to which the step's input is passed as a parameter. A step's output is a title selection that contributes to a filter run and hence to the entire filter expression that contains it.

The step's operator is drawn from a list of predefined keywoards which are known as filter operators.

Many steps require an explicit parameter, that further defines what the step is to do.

The suffix is additional text, often the name of a field, that extends the meaning of certain operators.

If a step's operator and suffix are omitted altogether, it defaults to the title operator.

Introduced in v5.1.23 Some steps accept multiple parameters which are separated by a , character.

Any unrecognised operator is treated as if it was the suffix to the field operator.

Filter operators can be extended by plugins.

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.