Why is it called an "underground" filter? If it's placed underground how do you see when the filter needs to be cleaned and why the clear filter bowl?

The filter itself should be installed upstream of the valves, in a separate valve box, so you can access the filter and clean it when needed (which is why the filter housing is clear, as you alluded to). The word "underground" in the title is there just to refer to the fact that this filter is designed for use as part of an underground irrigation system.