As Geoff says, that plate hides lots of wires BUT its the Volvo way of directing warm air to the feet of rear-seat passengers. Also of course it dries out the area condensation will settle if you either have a leak or like the car hot if frosty outside.
However, that hole which you seem to have removed the plug from is one of several deliberate ones along the floor sides - what they call tooling holes I believe. The proper jacking strengthenings are the smaller discs on the top-hat section outrigger a few inches along from your first photo. A genuine jack will engage on those so the cars side can be lifted safely without it falling off the jack. Personally I don't use them (except when miles from home with a puncture) - I use a trolley jack on the rear axle or front lower spring mountings plus an axle stand to be doubly sure as I don't want to risk getting under a car without.
OK re wrong section you wanted for your query, but it is applicable to the 164 model too, as that's what I run for fun.
P
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