What are all the triggers that allow playing a reaction spell, and can you play a reaction when an opponent plays a unit in their base?

Ruling: You can play reaction spells in response to anything that stays on the chain. Units and gear do not stay on the chain, so you cannot react to them being played. However, you can react to effects that resolve when a character joins the field.

Nuances:

• Units and gear cannot be reacted to because they don't stay on the chain
• Effects triggered by units (such as when a character joins the field) can be reacted to because those effects go on the chain