It is most likely an RMD. Once you reach the right age, plan administrators calculate these amounts for you. The monthly distribution is a key factor.
Required Minimum Distributions (RMDs) generally are minimum amounts that a retirement plan account owner must withdraw annually starting with the year that he or she reaches 72 (70 ½ if you reach 70 ½ before January 1, 2020), if later, the year in which he or she retires. However, if the retirement plan account is an IRA or the account owner is a 5% owner of the business sponsoring the retirement plan, the RMDs must begin once the account holder is age 72 (70 ½ if you reach 70 ½ before January 1, 2020), regardless of whether he or she is retired