You can use this operation to obtain a globally unique upload ID.
This upload ID is used to associate all parts in the specific multipart upload. You can specify this upload ID in each of your subsequent requests such as Upload Part, Complete Multipart Upload, and List Parts.
The key of the object for which a multipart upload is intended can be the same as an existing object. You can initiate one or more multipart uploads for one object.
An Initiate Multipart Upload request can contain multiple headers such as x-amz-acl, x-amz-meta-*, Content-Type, and Content-Encoding. The headers are recorded in the metadata of parts uploaded for the specific multipart upload.
This operation makes server-side encryption available.