I’m not sure of your question. The embedded media server (Twonky) in the lower end My Clouds will pass along SRT subtitle files (if they are present and contain the exact same name as the movie file) when the movie is streamed. Its up to the DLNA client to properly support SRT subtitle files.
If you are asking which My Cloud version is capable of running Plex, then see the following WD website link:
http://www.wdc.com/en/products/plex/Default.aspx
And see the Plex NAS Compatibility document: https://docs.google.com/spreadsheets/d/1MfYoJkiwSqCXg8cm5-Ac4oOLPRtCkgUxU0jdj3tmMPc/edit#gid=314388488. That document lists the following My Cloud units as being supported:
My Cloud Mirror Gen1
My Cloud EX2
My Cloud EX4
My Cloud EX2100
My Cloud EX4100
My Cloud DL2100
My Cloud DL4100
That document also indicates that transcoding media may not work or be supported due to the processor within the My Cloud not being capable enough.
The single bay My Cloud units do not officially support Plex being embedded within the My Cloud Firmware at this time. However, unofficially there may be ways to get the Plex Media Server running on the single bay My Cloud units if one replaces the existing OS with another. See the following link for more information:
https://community.wd.com/t/using-plex-media-server-with-my-cloud/94429/13
