I can confirm that Sharepoint will be adding at least one of these Custom Document Properties - ContentTypeID will get added when you load the file into Sharepoint. If you have custom document types in Sharepoint it is likely that other fields will be added. In our organisation we also use a third-party application to 'classify' documents and this adds six Custom DocProps by itself.

If I were you I would be using Linked Content Controls to simplify the customer's interactions with document metadata. Custom document properties are far more labour intensive to use and is not going to work well long term due to the ongoing education required for your users.

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Andrew Lockton
Chrysalis Design, Melbourne Australia