A pipeline author will occasionally need to set criteria on which a stage is "skipped" or applied.
This uses the "when" construct: (full docs here: https://jenkins.io/doc/book/pipeline/syntax/ search for "when" if you are curious).
- User can choose a stage an optionally provide a condition underwhich it will run (called a "when")
- This may be one of:
- * branch name specified (string)
- * environment variable name and its value (when that variable = value, it will be allowed to run)
- * a text box that allows an arbitrary expression to be entered