Hi Robin,
The reason the use of the ‘SUB_’ prefix is not mentioned in the best practices document, is because it is not a best practice :) It does not add anything.
If the prefix has a meaningful information about its use (ACT, BCo, DS etc) add it, otherwise I would adhere to “<ObjectName>_<Function>”
I did the exam at the start of this year somewhere and I seem to have used SUB_ at certain parts.
You could always try to ask the examinator present quickly before starting the exam if they accept the SUB prefix ;)
What they do is look at several areas (as discussed in the advanced course) and also look at overall impression as well. I doubt one prefix would be enough to make a severe impact on this