Intermediate now allows terrain to affect the Rng and Mov of units in addition to acting as a barrier, and introduces new kinds of terrain. It also introduces roll modifiers which affect your chance of hitting an enemy depending on the type of terrain they are occupying. There is also a cursory run-down on how to place units after moving.
The Intermediate ruleset is a logical extension of the Basic ruleset. The Basic rules always apply, but if an Intermediate rule conflicts with a Basic rule, the Basic rule is ignored.