Описание тега google-schemas
Google Schemas are metadata schemas that are added to emails, allowing contextual information to be displayed in Gmail, Google Searches, and Google Now. The schemas provide a context based on the categorized schema definitions from http://schema.org/, which potentially also allow the emails to be contextually represented in other search engines and products, not just by Google.
The way that the schemas are used by a product can vary from providing a summary of an email, to allowing users to reply to emails with customized actions, or even to collating schema-enhanced emails to provide an overall picture of upcoming activities in a users life.
Links useful for developers:
Google engineers monitor and answer against this tag, so it should be the first-stop for people wanting assistance with schemas.