This is the subject of much debate. Most Support Care schemes in England do use Section 20, though in a form that reduces paperwork and is specially designed to be as family-friendly as possible. So though in a strict legal sense placements over 24 hours come under LAC procedures, families do not have a perception of their involvement with support carers as being "in care".
Some authorities use Section 17 as the legal underpinning, and though this has not always been seen as fully satisfactory by inspection services, it has worked well in some instances.
So neither Section 20 nor Section 17 can be said to fit perfectly with Support Care, though both can be adapted in practice to be "good enough". Discussion with central government is ongoing on this issue, and everyone is hoping that some clear guidelines on the issue will be forthcoming.
NB At present all Support Care services in Wales operate under Section 20.