Give nearest entities glowing with a cap

How can I make something that gives the nearest entities glowing?

I know I can run a command that does something like effect give @a[insert tags] glowing X X

Which tags? I don't understand the dx and dy and dz thingies, I've tested them and can't get anything to work at all :/

PS I have the item and procedure ready, I just don't know what to put in the procedure.