With the Stash-triggered option (released in Bamboo 5.6), you don't get the option to select which of your Stash repos will trigger a build. We do offer this option with the product-agnostic "repo triggers the build" trigger, so it's kind of odd not to see it here.

Use case:
I have multiple Stash repos linked to my build. One of the repos receives changes several times per hour, which is more frequently than I want to run that build. I'd like Bamboo to ignore pushes to that repo, and start a build only when one of the other repos receives a change.