The last time I EVER messed with soft subs (Assuming this is refering to a video where you have to manually select your subtitles from a file containing the subtitles) was with HELLSING OVA 4 I think (Did Alucard crash the SR-71 in that one?). I was sorta worried about putting the video file and subtitle file on a disk for fear that the subs would corrupt but luckily such a fear was not needed as both the video and the subtitle file were inact. Only problem is that I had to find a RAW of OVA 4 in order to use the subs
Softsubbing is when subs are not embedded into the video file. Can refer to manually choosing the sub file or mkv. Since mkv (think of it like a box filled with videos audio font and subs) holds subs and video it is not embedded into the video calling it softsubs. If you downloaded the raw then added softsubs I would recommend not to do this. Mostly because you might not have the font that is required in the .ass unless they attach it but that can be buggy. Also unless you find the exact same raw they did the timing of the subs can be off. They can bleed scenes also and what not. Not recommended. If a group is too lazy to provide you with a muxed video then the translation is most likly bad as well. Wouldn't watch subs like that. Fastest is not always the best subs contrary to popular belief. Should do some research on translation quality of different groups and shows.