a operator should have a rule in place for mixing the fuel types. If JET A is less than 10% of the total quantity after refueling with JET A1 than the freezing point corresponds to JETA1. Apart from that for the polar flight the operator usually operates fuel with additives to lower the freezing point and determines actual one which might be below the published by the manufacturer e.g JET A1 for A 330 published in limitations sections is -47C but the actual might be -50C.
However if no info regarding the actual FP is given before the flight then it's as per limitations.
