[PhotoCoupler PCB hole]

I'm building a switching power supply and i've been studying commercial ones.

I've noticed that underneath the photo coupler, there is usually a hole in the PCB.
What is the purpose of this?

Thanks

[Crystal DataSheet]

Thank you

[Datasheet Request]

?

[Crystal DataSheet]

I have a rectangular object removed from an old modem, it has 4 pins and the markings are:

UT6-1B
40.320MHZ
USI 9318

I want to use this crystal as the clock of a microcontroller. I haven't used 4-pin crystals before and any help with finding the datasheet and/or usage would be appreciated. THx!