There are 2 basic types of captions: Closed Captions & Open Captions
Closed captions are thought to be embedded into the AV stream as auxiliary data or delivered as a sidecar file along with the AV stream. This type of captions are "formatted" on the fly by the (hardware or software) device used for playback. Customised appearance depends on the device as well - see above example for Mac and QT Player.
Closed captions can be turned on and off by the user and there may be multiple languages.
FCP closed caption support covers 3 format types: SCC, ITT, SRT.
Open captions are always visible, they are a visual part of the AV Stream like an inserted logo. They can't be switched on/off by the viewer.
But their appearance can be set by the creator. In FCP you can use "Motion Titles" to create those open caption types.
In your example the "Basic Title" is used - but it is disabled.
In the Timeline Index go to the Roles tab and enable "Titles" - the text will be visible. Click the "Basic Title" either in the timeline or the Timeline Index. In Inspector click the "Text" tab (the one with the multiple lines) and edit the appearance and the text. In the timeline edit the timing. Up from there you can use that title clip to create new "captions" - just copy the clip and paste at another time and change the text fields content.
A good idea will be to assign a role to these open captions. By default they have a simple "Titles" role, so there is no difference between "real" titles and your "subtitles". This may become a problem if you want to export a version of the movie without the "subtitles" - if you disable the "Titles" role all real titles will be disabled as well. Additionally the Timeline Index will allow to view either "real titles" or "subtitles".