Manning rule
The Manning Rule is a finance term based on the United States Financial Industry Regulatory Authority (FINRA) regulation, Rule 5320.
The Manning Rule prohibits a FINRA member firm from placing the firm's interest before/above the financial interests of a client.
For example, when a securities firm is holding a customer limit order (a limit order is an instruction to buy or sell securities at a certain price), the firm cannot ignore that order.
The firm cannot trade for their account using a price that would satisfy the customer's limit order without executing the customer limit order. The rule is applicable both in normal trading hours and in the extended hours trading sessions.