From ATS 7 rules....
Minimum fill quantity
The system allows a minimum fill quantity to be entered for orders. A minimum fill order seeks to execute a stated minimum quantity of securities on entry, failing which the order will be expired. If the minimum quantity is executed, then the remainder will be treated as a regular order (i.e. minimum quantity will no longer apply).
All order types (i.e. Market, Limit, Stop, Stop Limit) can be submitted with a minimum fill quantity specified. If a TIF (Time-In-Force) is submitted for a minimum fill order, the TIF will only apply on the remainder of the order, after the minimum quantity is executed.
Example: A buy limit order of 500 shares with a minimum quantity of 100 and a TIF of GTD is submitted to the order book. It executes the minimum quantity against a contra side order and the remaining quantity of 400 will be added to the order book with the GTD qualifier applied.
The exception will be an FOK min fill order. This will be treated as an FOK order and not a minimum fill order.
Refer these links to see changes in ATS 7 and Complete ATS Rules. If you have read already please just ignore.