For a list of other such plugins, see the Pipeline Steps Reference page.
By default, Pipeline builds can run concurrently.
The milestone step forces all builds to go through in order, so an older build will never be allowed pass a milestone (it is aborted) if a newer build already passed it.
In general this step grants:
An optional ordinal for the milestone. It is autogenerated if not explicitly set.
Setting explicit milestone ordinals grants that each milestone can be univocally identified across builds even when script changes are made during a previous build. If you plan to add or remove milestone steps while there are running builds and trigger new builds before the previous ones finished, then you should set explicit milestone ordinals. Otherwise, let the step autogenerate them as it runs.
A label for the milestone. It's shown in the build log metadata.