In order to obtain a DBS Disclosure, it is the job role that is of significance, e.g. carer, tutor or sports coach etc. The word volunteer is immaterial and is not accepted by the DBS.
There is a button to press on the position page if the role is for a volunteer, which automatically alerts the DBS to that status in order to check for the regulated activity eligibility and to ensure it is a free DBS check.
Volunteers are always deemed to be supervised under the Protection of Freedoms Act so organisations must ensure they have made adequate provisions when working with with children or 'vulnerable' adults.