As per SEBI's circular, brokers must transfer funds from your Zerodha account back to your bank account once a quarter, which is also called quarterly settlement.
However, brokers can retain up to 225% of End Of the Day (EOD) margins if you have an outstanding position on the quarterly settlement date. The retention statement explains the reason for retaining funds. The regulators specify the format of the retention statement. You can view the format under Annexure 1 of the NSE circular.