Uploading ore replacing of files in the DAM Center is done as a sequence of steps which gives the following advantages:

Support for uploading any file size.
Ability to resume broken uploads from the client.
Add metadata before or during actual file data is sent. For large files, this is an advantage

The steps in order:

Initiate upload by calling method IntiateUpload
Invoke method UploadFileChunks to send the data in chunks.
If uploading, Invoke method UploadAsset to finalize upload
if replacing, invoke method ReplaceAsset to finalize replace

After UploadAsset or ReplaceAsset, the job system will begin all necessary transcodes and other workflow defines steps. As stated, it is also possible to work with the asset before the actual file has been uploaded. This is possible because all the metadata service endpoint needs, is the itemId. You get that already from the first step in the upload process.