If you only have a small limited number of different fields, it is easiest to pick from the built-in metadata fields. Go to Insert > Quick Parts > Document Property > pick any one you like. You can make copies of this and paste into multiple locations. When you change the value of one, the others will automatically change. The drawback is that all these entries are going to be plain text Content Controls.

To add a linked Content Control of any type (including a ComboBox): Go to the Developer tab > XML Mapping Pane > Find your preferred Custom XML and Node > Right click it and choose > Insert Content Control > choose the type you want.

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Andrew Lockton
Chrysalis Design, Melbourne Australia