A d-stop pronunciation is any pronunciation that is shown in Webster’s Third with a 'd' followed by a dot, as d• for a word that has a ‘t’ in it. Letter and butter are examples of words whose first pronunciations have a d-stop in them. They are pronounced <LED.uhr> and <BUHD.uhr>. In these cases, both have an additional pronunciation that shows a ‘t’, as <LET.uhr> and <BUHT.uhr>.
There are a surprising number of words in Webster’s Third that have a ‘t’ in the word with only one pronunciation shown, and that pronunciation has the d-stop in it. Parameter is a common word that can only be pronounced <puh.RAM.uhd.uhr> according to the dictionary. The pronouncer at the National Bee thus will only give that single pronunciation and will not pronounce the ‘t’ in parameter as a ‘t’ sound.