Understanding CI_PIPELINE_SOURCE=merge_request_event in GitLab CI/CD Pipelines: How Merge Request Events Work

When you create a merge request in systems like GitHub or GitLab, it’s common for these platforms to run pipelines specifically associated with that merge request. Here’s what happens under different conditions:

Standard Pipeline (as seen initially): When the pipeline is triggered by actions on its branch—such as a push event (CI_PIPELINE_SOURCE=push) like in your first example, it runs jobs associated with that specific commit or range of commits from within that branch itself:
```
rules:
  - if: $CI_PIPELINE_SOURCE == 'merge_request' && ($BRANCH_NAME =~ /^(feature|hotfix)\/.*/ || isMaster)
    # ... rest of pipeline configuration goes here.
```
This ensures that jobs are executed when new commits come in on feature or hotfix branches, as well as the master branch if certain conditions apply (e.g., you’re merging to release a stable version). The links provided would take users from seeing their job details within this context directly related to its source code lineage and associated features/issues under development therein (bar/- in your case).
Merge Request Pipeline: When the pipeline is configured for merge request events, it will run jobs specifically linked with that event rather than committing ones from a branch (as shown next):
```
rules:
  - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
    # ... rest of pipeline configuration goes here.
```
The primary difference is that the jobs are related to what you see in your merge request, not necessarily where they originated from within a branch on their own repository history (bar/-/tree). When clicking through these pipelines and job details as depicted with images taken directly off Stack Overflow galleries (with hyperlinks provided), one will notice that the content of those jobs is more reflective of what’s been merged into your merge request, rather than existing within a branch’s history.
Epic or Project-Wide Pipeline: Sometimes projects also have pipelines at an epic/project level which are triggered by specific events across the project (like tagging releases). These would not be directly tied to any one merge request, but rather serve a broader function within your development workflow for that particular release cycle or set of features.

In summary: when dealing with standard branches and their pipelines, you’re concerned primarily about commits on those specific lines; however, during the lifecycle of creating changes via merging (a merge request), there is a distinct separation between what has just been merged into your repository (foo in this case) versus previous state or development within separate feature/hotfix branches.

The provided Stack Overflow images help visualize these differences—from job details to pipeline summaries, and how they all interact with the branching structure of a typical Git-based version control system like GitHub or GitLab when merge requests are in play. The post on your own Epic page indicates active discussion around this topic within the wider community at large who might be seeking clarity—or looking to standardize practices for setting up such workflows effectively across similar projects, with a strong emphasis placed upon understanding these nuances and their practical implications during code review processes.