1. Maintenance Margin violation: In an account, the Equity with Loan Value (ELV) must always be greater than the Current Maintenance Margin Requirement (MMR) on the positions that are being held in the account. The difference between ELV and MMR is Current Excess Liquidity; therefore, an easier way for some people to monitor their account is to remember that the Current Excess Liquidity in their account must always be positive. If the Current Excess Liquidity in an account goes negative, this is a maintenance margin violation.
2. Reg T violation: In the Balances section of the Account Window there is a figure titled Special Memorandum Account (SMA). The US Fed has an enforcement period for this account; 15:50-17:20 ET each trading day. During this window, the SMA balance must be positive. If the SMA is negative at any point between 15:50 and 17:20 EST, this constitutes a Reg T margin violation.
In the event of a margin violation, the account is subject to automatic liquidation on a real-time basis. Liquidations are accomplished with market orders, and any/all positions in the account can be liquidated.