A feature of Odata batch request formatting is changesets.
This is one or more nested boundaries within the batch as documented here:
http://docs.oasis-open.org/odata/odata/v4.0/errata03/os/complete/part1-protocol/odata-v4.0-errata03-os-part1-protocol-complete.html#_Toc453752316

This is a requirement to support Azure Storage/Cosmos Tables API.