This would be a question for either Jason Livingstone or Giovanni Galvez of Telestream to answer definitively.
My understanding of .scc is that it supports EIA608/Line21 only and that people still ask for them associated with HD out of ignorance that it is not 708 compliant. Some caption authoring software will allow you to import a scenarist file to generate EIA708-wrapped stand-alone files, such as .cap, .aaf, and the soon-to-be more widely recognized .mcc as the VANC-compatible donor file.
The fact that several NLEs and disk-authoring software allow the import of .scc glosses over the reality that while the text and time-stamp are just fine, all the other metadata isn’t, including the extremely important requirement (being enforced more rigorously now) for custom positioning on-screen. This is not part of the 608-Line21 technical description, but it is a major feature of 708. The other issue is that scc is tied to 29.97 and if you try to make one that is some other frame rate, the authoring program will warn you (or should warn) that you are not EIA-compliant anymore, and if your work is gong to be QC’d for broadcast, either be prepared to justify the export or wonder if the deliverables are being grab-bagged from the internet and they are just adding more” important-sounding stuff” to the document.
https://www.3playmedia.com/2015/08/11/whats-the-difference-between-cea-608-line-21-captions-cea-708-captions/
jPo
“I always pass on free advice — its never of any use to me” Oscar Wilde.