wrote

I just installed a sink drain.

The drain has two retangular holes (about 1" by 1") in it just below
the top surface
of the sink and above the underside surface of the sink (above the
undersink gasket)

What are the purpose of these holes? Seems like they just provide a
cavity between
the outersurface of the drain and the inner surface of the hole in the
sink where the
drain fits. Drain water must accumulate in this cavity, I would think?

There should be an "overflow" slot in the sink up near the top edge.
The water that goes through the overflow has to be able to get into the drain
pipe, even if you have the plug in the basin, so it comes in the sides.