This is stated in the FAQ of the Warhammer 40,000 Rulebook
Q. Can you clarify what the difference is between ‘wholly within’
and ‘within’ for rules purposes?
A. If a rule says it affects units/models that are ‘wholly
within’ then it only applies if the entire unit/model is
within. If it just says ‘within’, however, then it applies so
long as any part of the unit/model is within.
For example, units gain the benefit of cover if every
model in the unit is either on or within terrain. So long
as all the models in that unit are either on or partially
within the terrain, they gain the benefit of cover.
KFF has a bit of a different wording "entirely within" instead of "wholly within" but it's the same really, so all of the model base needs to be inside the KFF bubble. (i miss the old way)
But don't KFF have exactly the same wording as the cover rule ?
"If a unit is entirely on or within any terrain feature..."