Docs feedback: Unclear when CI_PIPELINE_SOURCE is "push"

In the section https://docs.gitlab.com/ee/ci/jobs/job_rules.html#ci_pipeline_source-predefined-variable, push has the following description:

For pipelines triggered by a Git push event, including for branches and tags.

When exactly is a "Git push event" triggered? Is that only triggered when pushing directly to a branch, or also when merging a merge request using the Gitlab UI?