The vintage space video Missions we Lost When Apollo was Cancelled after
01:25 describes classification scheme for then-future Apollo missions:
As Apollo took shape in the background, as Mercury and Gemini solved all kinds of problems living and working in space, and so too did the outline of how the missions would proceed. They were all given a letter to denote the mission type.
E missions would be manned CSM and LM test on a simulated lunar mission. This would see the spacecraft in a highly elliptical orbit, hitting a peak apogee of 3500 nautical miles.
Question: Besides the Earth looking smaller out the window at a roughly 6,500 km altitude, in what ways would this simulate lunar missions? I'm guessing that something about a longer burn might be part of the answer, but there many be more.
The video is chock-full of factoids, some possibly relevant but I'm not very conversant in Apollo-speak. Those that are may appreciate the video!