Fields
From Starfleet Commander
Fields are the amount of space a planet has to build upon. You can continue building on any planet until you have used up all fields.
Your home world starts with 163 fields, but colonies get a random number. More fields are always better.
Fields are used whenever a building is built or increased in level. One field slot is used per level, so a level 3 Shipyard would consume 3 fields. Defenses do not use up field slots.
