A Temporary Reality Number is a number that we create on this site to keep track of things which do not have an official Marvel reality designation yet. For instance, the Legion television show that came out recently did not have an official designation, so we give it number Earth-TRN620.
We do that so users will be able to create and edit articles while we are waiting for the real number to be released. When it is, it is easy to find all of the things related to a TRN and change them.
Thank you for assisting me on fixing the new page for Guardsmen Super-Soldiers (Earth-616). We need to place it on the Google website, so when people type it, they will be able to go to the page that we completed.
We use that tag on several things that are supposed to be tie-ins to movies and such, like tie-in comics or video games, that aren't necessarily canon to what was established in the source material and in the canon follow-ups to it. In regards to that comic, much like the similar Avengers: Age of Ultron: Episode 0 #1, it falls under "semi-canon" section on Earth-199999#Notes in regards to MCU-continuity.