It gives you the freedom to convert dfxp (itt and others), sami, sbv, srt, stl, vtt, untimed text etc into either Titles or ITT captions projects with full role support for both types.
Simply select the target type and your subtitle source.
While it is technically not a problem to include CEA it doesn't make sense. FCPX doesn't support CEA 708, means not all languages are supported. Additionally from my experience 99% of subtitle sources are not "CEA safe" and will result in a project with 100s of errors.