Schemas define basic information allowed and required in an ATML file.  Schemas also provide validation of data and hints for data entry.

Here is the official site to find the most recent schemas:
http://standards.ieee.org/downloads/1671/

Here's a link to all the Phase 3 demo ATML schemas:
http://grouper.ieee.org/groups/scc20/ATML/Demonstrations/Phase3/Baseline%20ATML%20Schemas/