Trade
Trading of resources is a way to increase the attitude between two races, as well as provide each race with a resource that it needs to construct various buildings.
Trading resources
Spacefaring races can exchange resources for a net gain in attitude towards each other in the Commerce Partner category.
Tariffs
Normally when you have a trade route between two planets, each side gains an attitude boost toward the other in the Commerce Partner category. Imposing a tariff from this planet to the other planet means that the other planet won't gain those attitude improvements toward this planet, but instead this planet gains a monthly boost to its Economic RCI value. Trade tariffs are one-way and universal from one planet to another (aka they are not per-trade route). So unless the other end also establishes a tariff against this end, the attitude bonus keeps happening over here, AND we get points on the economy scale. If a planet changes hands, then all tariffs are wiped out on that planet.
Trading tech
One race provides tech to another for an attitude increase. Occurs because of player interaction, or due to being part of the same alliance.
