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

My mother in law is 76, receives California Public  Employees' Retirement System money monthly.  is this considered RMD?  In box 11 it says 1st year of desig Roth contrib.

The answer is very vague???

Since you are over the age where required minimum distributions (RMD) are required, you should answer "yes" to the question regarding your distribution being a required minumum distribution.