Requisition Points (RP) are the currency used to purchase some additional classes (Neptune, Hydra, Paladin, Berserker), armour, weapons, talents, ordnance and relics from the requisition tree. You are allowed to examine an upgrade prior to purchasing but RP can not be reimbursed should you make a choice you later regret, such as buying a high level gun too early. i.e. a gear level 9 plasma gun when your total gear limit is 10.
They can only be earned through the completion of a mission. The number of RP awarded depends on the mission itself, as some will offer poor rewards and others will be generous, in all cases the base reward is only shown at the end of a mission. The base reward is modified by the game difficulty (as higher difficulty tiers payout more than easier tiers) and the cumulative of the multiplier (x1.00-1.40) earned as your Mission Rewards. Secondary Objectives and escorted humans will not award any bonus to the amount of RP you receive.
The incentive for earning as large a multiplier as possible is that the cost to acquire everything on the requisition tree is at least 18,590 RP. For comparison a successful campaign will reward as little as 5,105 RP, depending on the missions you attempt.