Regulation D imposes reserve requirements on certain deposits and other liabilities of depository institutions solely for the purpose of implementing monetary policy. It specifies how depository institutions must classify different types of deposit accounts for reserve requirements purposes.
The idea is to have money market and savings accounts used for longer savings of money. Because of this, there is a limit of 6 withdrawals per month*. The goal is to get people to use checking accounts for payments of bills.
This applies only to convenient transactions. Visits to the bank branch or ATM withdrawals are without limit.
*The Federal Reserve suspended this in April 2020 although individual banks may still adhere to the policy.