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How are executions allocated when an order receives a partial fill because an insufficient quantity is available to complete the allocation of shares/contracts to sub-accounts?…

If you do not modify an order in between partial fills and the order is executed within the same trading day, then the commission will be applied as if the order were a single fill with regard to commission minimums.

However, orders such as a GTC (Good-til-Cancel) order that persist overnight will be considered a new order the next day for the purposes of determining order minimums…

Many exchanges charge fees to their members for orders that are cancelled or modified, and IBKR charges certain related fees to its customers to help cover these exchange fees. Please note however, that the Modify/Cancel fees listed are not intended to be a direct "pass-through" of exchange fees. The Modify/Cancel fees that IBKR charges to its customers may be higher or lower than the fees that an exchange may impose on IBKR. IBKR may charge Modify/Cancel fees to its customers for orders sent to exchanges that do not impose such fees, or vice versa…

Modified orders will be treated as the cancellation and replacement of an existing order with a new order. On certain exchanges, this may have the effect of subjecting modified orders to commission minimums as if they were new orders. For example, if an order for 200 shares is submitted and 100 shares execute, then you modify the order and another 100 shares execute, a commission minimum would be applied to both 100 share orders. Orders such as a GTC (Good-til-Cancel) order that persist overnight will be considered a new order the next day for the purposes of determining order minimums…