'CUP' means Copper Units of Pressure. It's just a method of calibrating pressure, literally, by means of a wee copper cup and measuring how much it was crushed. This stuff is explained(in plain American English) in the reference chapters of your manual.
"...a single shot which I hear can take Marlin loads..." It's not Marlin loads. It's Modern Rifles or Lever Action loads. That single shot you heard about would be a Ruger No. 1(no longer available, new, in .45-70. Starts at about a grand, used.). A Ruger No. 1 is far stronger than any Marlin lever action will ever be. A Ruger No. 1 can handle .45-70 loads that are close to .458 Winchester with no fuss. A Marlin or any other lever action will be blown to smaller pieces(the gasses usually blow out the bottom of a lever action. Might completely destroy one.) with those loads. Your CVA cannot use those loads.