From the Warhammer 40,000
Main Rulebook FAQ:
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.