Fill policies in MT5 and the order that never completes

[Amanhasnoname]

MT5 offers three fill policies: fill or kill, immediate or cancel, and return. They behave differently, and the wrong choice causes confusing outcomes.

Fill or kill means the whole order executes at once or not at all. Immediate or cancel fills what is available and cancels the rest. Return allows the remainder to stay on the book as a pending order.

For scalpers, the right option depends on the size and the instrument. Small orders on liquid pairs rarely hit limits, so any policy works. Larger orders on thin pairs can partially fill, and the policy decides whether the leftovers hang around.

The broker's server decides which policies are allowed per symbol. An order sent with the wrong policy gets rejected with an error that is not always clear.

Check the symbol specifications for allowed modes, and set the platform default accordingly. If using automated tools, make sure the code handles the case where the order returns partially filled.

Which fill policy has worked best on your account, and did a wrong setting ever cause trouble?