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1.==>In the "Add Operating System Image" phase, does the .wim file remain on the remote network share, or is it still transferred to headquarters?
The .wim file on the remote network share will be transferred to headquarters via VPN, unless there is a primary site on the remote location that can manipulate the content individually.
When we create a new package from the administrator console that hasn't been distributed to any DPs yet:
If there is only Distribution Point or Secondary site on the remote location, after the administrator creates the package in the console on local primary site, it will transfer not only the metadata information of the .wim file but also all the .wim package content to the content library on the local Primary site server. So in the "Add Operating System Image" phase, the .wim file on the remote network share will be transferred to headquarters via VPN. All the package content must be add to the content library on the Primary site server before we can do relevant content action, such as distribute, update or redistribute.
2.After the .wim package is created completely, we can then distribute the content to distribution points by the Distribute Content wizard in the Configuration Manager console. This will transfer all files in a package to the target distribution points, excluding those which are already present in the content library of the DP as part of another package.
For more information about content library and content distribution, please refer to:
Package actions in content distribution
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