In Demand Management, Demand Templates are used for different scenarios to validate uploaded files, such as for particular Part Categories. Then when uploading a demand list, the template can both populate and/or validate your list's parts to the data found in Part Master via that demand template's Part Category specifications.
Therefore, when uploading a demand and a populating rule for that column from Part Master has been set up, then data will be prioritized coming from the Part Master if data exists for a specific IPN. If population rules are set up, but that field is empty in Part Master, then the demand will contain only the data within the upload file itself.
More information for admins regarding the configuration can be found within the articles regarding Demand Templates Management.