Official rule text
93.4 A player can spend a trade good in one of the following ways: a In place of spending one resource. b In place of spending one influence. However, trade goods cannot be spent to cast votes during the agenda phase. c To resolve an effect that specifically requires that a trade good be spent.
View official source
Source: Living Rules Reference and FAQ - Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 93.4