I've never seen a legal document stating exactly what is meant by supervision, therefore it is always open to interpretation.
Then start with the ANO Schedule 7. This is what it says for an AFI and there are still some left:
Assistant flying instructor's rating
(2) (a) Such instruction must only be given under the supervision of a person present
during the take-off and landing at the aerodrome at which the instruction is to
begin and end and who holds a pilot's licence endorsed with a flying instructor's
rating entitling the holder to instruct on an aircraft of the same type or class as
the aircraft on which instruction is being given.
The document that Bose is looking for is a CAA TrainingCom (now out of date) which included the above reference together with the JAA one which was less specific. The latter was removed from the ANO in Aug 2012 and replaced by the legal reference for an EASA rating as Curtis has stated above.