Although you must still undertake a DBS (formerly CRB) check for staff that have lived overseas, it is necessary to undertake further checks with the country where they lived. These checks may include certificates of good conduct from local police forces or embassies, follow ups to anything that is provided should be made (e.g. call the organisation that sent the document and/or make checks through embassies etc).
Who needs to complete this section?
Volunteer is at the school's discretion. If you had a person who lived outside of the UK for five years and wants to volunteer, the DBS would not cover this time out of the UK so it is at the school's discretion.