The icon and the 1 rating are intended to be used by handicapped cacher to aid in searching for caches.
The simplest way to search for caches is to go to the geocaching.com home page enter your zip code and search. If the second rating is a 1, the cacher can look at that cache and see if it is of interest and decide if it is within their capability.
Premium members can search using icons. The handicapped icon lets handicapped cachers can search for caches they might want to find. Again, cachers, handicapped or not decide which caches are of interest.
If a cache has a 1 terrain rating, it should also have the handicapped icon. If you think handicapped cachers should consider seeking your cache, use the 1 rating and handicapped accessible icon. Otherwise, the terrain rating should be greater than 1.
The farmer gave permission to place the cache in the field, but the bull charges.