Specifies the Groovy filter expression that restricts the subset of the combinations that the downstream project will run.

See the "Combination Filter" field in a matrix project configuration page for more details about the environment the script runs in, examples, and so on.

What you specify here gets expanded by variables of the triggering build, which allows you to dynamically control the subset of the triggerred job. For example, if you trigger job BAR from FOO with label=="${TARGET}" in the filter, and if FOO defines the TARGET variable and FOO #1 sets TARGET to be linux, then the triggered BAR #3 will only select the subset of combinations where label=="linux" holds true.

Note that the variable expansion follows the ${varname} syntax used throughout in Jenkins, which collides with Groovy string inline expression syntax. However, Jenkins variable expansion leaves undefined variables as-is, so most of the time your Groovy string line expression syntax will survive the expansion, get passed to Groovy as-is, and work as expected. If you do need to escape '$', use '$$'.