I just stumbled looked for the same documentation and found this question, so I thought I’d share the best answer I found:
CI_PIPELINE_SOURCE values are defined in the
merge_request_event is defined as follows:
For pipelines created when a merge request is created or updated. Required to enable merge request pipelines, merged results pipelines, and merge trains.
As per your question
In particular I would like to know if the value
merge_request_event is assigned, if the pipeline was triggered, because a merge-request was merged into the current branch.
The definition does mention that it’s the value for when a merge request is updated, and I guess a merge request whose branch has been the target of a merge counts as that, so I’d say the answer is yes.