include:component is usually what you want now, you can version your components (Semver), add a nice readme and it is somewhat integrated in the gitlab UI. Not sure about the other include: ones, but you can also define inputs for component and use them at arbitrary places like template variables.
Since the integration is done statically, it means gitlab can provide you a view of the pipeline script _after_ all components were included, but without actually running it.
We are using this and it is so nice to set up. I have a lot of gripes with other gitlab features (e.g. environments, esp. protected ones and their package registry) but this is one they nailed so far.