Transmitter cover tightness

I was led to believe that there were ATEX requirements for screw cap covers on transmitter electrical housings to be screwed on hand tight to allow any heat generated by a fault inside to cool down through the threads and escape. I recall depth gauges to be used in the past..

Now I read in most vendor literature that the covers should be screwed on so that metal meets metal.. What does this mean exactly?

Should the technician use leverage to tighten the covers?

Thanks
Gray