Microsoft's site that publishes independent audit reports (SOC 1, SOC 2), certifications (ISO 27001), documents by industry and region, and resources on how Microsoft cloud services protect data.
In SC-900 it is the answer when a customer needs to download Microsoft's audit reports. "My Library" lets you save documents and get notified when they are updated.
Service Trust Portal is a concept that can show up on the exam. Check the full guide to see it in context.