Ratings for Difficulty and Terrain (D/T)
Ratings for difficulty and terrain have changed over the years and can be very subjective. They can even change from location to location. So what is a Terrain 3 in 2002, may now be known as a terrain 4.5. And a terrain 4 in Auckland, could be a terrain 3 in Perth.
Use the D/T on a cache as guide. It will give an idea of what to expect.
Below is the D/T guide from geocaching.com that may simplify things a bit for you.