Income rate.
Credit income rate is influenced by planet population, alleigance, longest trade port chain, and number of trade ports.
Resource income rate is influenced by alleigance, number of constructed extractors, number of captured neutral extractors, number of refineries (or trade ports using Resource Focus and what % bonus to extraction rate RF provides for the Advent), and researched increases.
All income rate is decreased as your maximum fleet capacity increases (0%/9%/19%/28%/38% (?)/47% (?)/56%/66%/75% taxed).
