The word restricted simply means you need special permission to enter a restricted place. The trekking route which needs special permits or documents to trek through is a restricted area . You can get this type of trekking from the govt of Nepal, Kathmandu (Nepal Tourism Board & Nepal Immigration). There must be a minimum of 2 people to issue this permit. Upper Mustang and Upper Dolpo among a few others are marked as restricted area trekking. Only a few foreigners had visited this part of the world because it is perhaps one among the remote trekking trails.