The lack of clarificatioin is very unhelpful
latest update is here
https://www.gov.uk/guidance/claim-for-wage-costs-through-the-coronavirus-job-retention-scheme
D&C
They are either furloughed - doing absolutely no work for the company - or not.
That advise is now outdated - certain volunteer work can be done as clarified by yesterdays update
If your employee does volunteer work or training
A furloughed employee can take part in volunteer work or training, as long as it does not provide services to or generate revenue for, or on behalf of your organisation.
However, if workers are required to for example, complete online training courses whilst they are furloughed, then they must be paid at least the NLW/NMW for the time spent training, even if this is more than the 80% of their wage that will be subsidised.