The Reinforced Reception Security Code is a temporary security code used to unlock a main gate. It is one of four codes required to open the gate during the "Locked Gate" Blue Gate map condition.
You can find this code in lootable containers, such as bins, located within the Reinforced Reception area on the Blue Gate map. At least one code will appear per round during this condition, and sometimes more.
To use the code, head to the Gate Control Room on the Blue Gate map. Once inside, interact with one of the four terminals. Submitting a code consumes it and activates the terminal which helps to open the Locked Gate.