In StarCluster 4, base skills cannot progress above rank +2. if you raise the rank of a base skill to +3 or beyond, it becomes a narrower Field of Expertise within the skill. You can name the field yourself, or choose one of the example field names. Example: Science+2 becomes Science +2 (Biology+3).
StarCluster has always had a large, center defined, and overlapping skill set. Now it will have a small, center defined, and overlapping skill set.