List of predefined variables that are passed *unchanged* to the bridge job

So the documentation doesn’t say much about bridge jobs except that they are “a little different to regular jobs”.

What I’m looking for is list of predefined variables (such as those with CI_ prefix) that are passed from upstream job to the bridge job.

For example, inside the bridge job the CI_COMMIT_REF_SLUG variable is the same as in the upstream job, but eg. CI_PIPELINE_ID, CI_JOB_ID or even CI_PROJECT_ID are not.

I need this information because I need some unique, per-job/pipeline variable from the upstream job which I can use in

downstream 
    trigger: 
        branch:

that is because the branch I’m pointing to is created in the upstream pipeline (using API) and it should be unique per pipeline, thus even though I have CI_COMMIT_REF_SLUG at my disposal it doesn’t necessary have to be unique.

Thanks