When the interaction is replied to, Reward sites will fire and items potentially selected based on whether the given unlock expression is TRUE.

You can potentially choose one from multiple replies when replying to an interaction: this is how you get either/or rewards from vendors.
Type: object
Object Properties
The rewards granted upon responding to the vendor.
Type: int32
The localized text for the reply.
Type: string
An enum indicating the type of reply being made.
Type: int32