"Highlands" is a broader term designating -- for lack of a simpler explanation -- the white parts of the lunar surface as seen from Earth. It's basically anything that's not a mare. Maria make easier landing sites due both to orbital inclination and terrain. With increasing proficiency in piloting comes the ability to land in more far-flung areas of the Moon, and among rougher terrain. The J-mission landing profile was steeper and faster and require more from both pilot and machine to make a safe landing. Hence the highlands exploration came later in the program.
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