Terrain features may appear at varying degrees of likeliness. A good base chance is unlikely but may increase based on adjacent sectors. Also there may be reasons to favor certain features over others. For instance in a rocky desert you might roll twice in favor of crevices and cliffs. You might also roll twice if adjacent sectors have certain features that could meld into another particular feature.
Rare features may sometimes appear. Some may be more common than others or as common as terrain features depending on what kind of world you are on. A good base chance is very unlikely. Additional rolls may be needed to determine specific details of the feature. "Other" may represent something unusual or something specific to this particular planet. For instance to start with there is likely to be ship wreckage scattered around the planet.
(1d6) Rare features 1. City/City Ruins 2. Crater 3. Volcano 4. Other 5. Other 6. Other