Extremely thin Oxygen / Nitrogen atmospheres.  Specific gravities are within
0.8 to 2.6 Earth normal.  Climate patterns are violent with temperatures
from 45 to 400 degrees Celcius.  Full life support required for colonization.
Zero workable soil and harsh conditions make Organics production impossible.
Good trace elements for equip but conditions make production a gamble at
best.  Excellent Ore production possibilities as material is often ejected by
volcanic activity and found on the surface.  Very dangerous for colony growth
as unstable planetary crusts often lead to the complete loss of a colony.
The Federation has been known to use Class H planets for defense of key
sectors due to their large Ore base.