I think this is a bug in description, that just should be - "speed limited by crossing"
Each crossing has two speed limits - one for each waytype. The crossing can limit the top speed of both ways. I don't know the rules in UK, but i Czechoslovakia, you have to slow down to 50 km/h on rail crossing, even if outside the town where the limit is 90 km/h. Also level crossings are forbidden on tracks faster than 160 km/h (which might be the reason why we do not have higher speed tracks anyway). And if the crossing is not safeguarded by light signals (only a warning sign), then the speed limit on road is 30 km/h, and 60 km/h for trains. Anyway for pak128.britain-ex, the fastes crossing allows 50 km/h on road and 200 km/h on track.
The implementation is standard has been changed recently, and the speed limits forbid building level crossings over higher speed road/track than indicated.