YAML can be configured with multiple repositories through checkout task. However, trigger in YAML could not be configured to enable trigger for certain repository only.
If a single repository is used to manage YAML Specs for multiple plans, a Specs change in a plan will trigger normal builds for all the related plans (if there are 50 plans affected, 50 builds will be added to the queue). This will flood the queue and will take some time for the agents to clear the queue.
Configurable trigger setting in YAML so that only certain repository change will trigger the plan
Configure exclude function at Administration >> Linked repositories >> <repoName> >> Change detection options to exclude file pattern "bamboo-specs/.*"