The SMPTE-TT CEA-608 Captions export option is a derivative of the SMPTE Timed Text specifications which strictly follows the guidelines set by "SMPTE RP 2052-10:2012" describing how standard CEA 608 captioning data must be converted into a SMPTE-TT compliant XML. The SMPTE-TT CEA-608 Captions option is intended solely for the purposes for Closed Captioning and should not be mistaken with the more general SMPTE-TT Subtitles export.
The single thing you need to consider while importing SMPTE-TT files, is the position of the text on screen. It will be determined according to the Vertical/Horizontal Safe Area width specified in the SMPTE-TT file itself.
However, if this information is not present you can specify it in the dialog as explained below:
The Vertical and Horizontal width define a region on the screen which guarantee that the whole text will be displayed and width of 80% is considered default because it equals the CEA-608 requirement for positioning the text in 32x15 grid.