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At their core, a Blueprint Spline Component is just a path for you to define and use positional data. You can use it to move Actors (or other Components ) around the world, or place a series of Actors (or other Components ) along the spline. They are fully editable in the Blueprint Viewport and in the Level Editor, with the ability to add/remove/duplicate Spline Points, change their tangent types, and even animate them on tick. Further, they are also editable using the Blueprint Construction Script , taking in edits made in the Blueprint Viewport or Level Editor, and mofifying them further.
Blueprint Spline Mesh Components have a completely different use case. These deform a single Static Mesh along a two point spline. You cannot add more Spline Points to a Blueprint Spline Mesh Component, but the two points are completely controllable through Blueprints.
While their use cases are very different, they share the same procedures for adding them to a Blueprint, and use the same editing tools.