From the paper:
RHUBARB
A small offensive operation, using cloud cover, carried out by 1 or 2 sections, the primary object of which is the destruction of enemy aircraft. Rhubarb operations may also be directed against suitable predetermined ground targets, in accordance with current Rhubarb Instructions. Aircraft should approach ground targets at 0 ft. climbing only to make their attack. Cloud cover must be available in which to evade any enemy fighters encountered or to avoid intensive flak. Cloud 10/10 at 1,000 - 3,000 ft. with good visibility provide the most suitable conditions for Rhubarb operations. If weather proves to be unsuitable when approaching the enemy coast, the sortie should be abandoned. Pilots undertaking Rhubarb sorties must be carefully briefed. Navigation at 0 ft. must be accurate.
RANGER
Similar to operation RHUBARB but with deep penetration.