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A player's mobility percentage refers to how much their ability to get around had been affected by the cargo they're carrying and/or the armor they're wearing.
It also refers to how much the player's stamina is affected when performing stamina related actions. For example, if a player has a mobility of 75%, he or she will see a 25% increase in stamina usage, as well as a 25% reduction in stamina regeneration (needs verification).