Fill policy: why an order gets rejected even though price is there

[Shadow Trader]

MT5 has fill policies: Fill or Kill, Immediate or Cancel, and Return. They decide what happens when the full size isn't available at the requested price.

Fill or Kill demands the whole order or nothing. Immediate or Cancel takes what's there and cancels the remainder. Return lets the remainder rest as a pending order.

For small retail sizes, the distinction often goes unnoticed. But on thin pairs, or in fast markets, partial fills appear. A 5 lot order might fill 2 lots at the requested price and 3 at a worse one, or only 2 at all.

Checking the symbol's allowed filling modes in the specification sheet takes seconds. Setting the order to match avoids a whole class of mysterious rejections.

Partial fills also complicate stops. If only part of the position is open, the stop and target need to match the size actually filled.

How was it discovered the last time a partial fill left a position half-protected?