Similar to Microsoft Word DocProperties where you're able to use update a DocProperty value as metadata, and its value gets update in the whole document.
Does Google Docs have something similar?
(I'm planning on modifying said variable's value programmatically from the outside of the document via a script.)
Best Answer
Short answer
Yes, Google Documents, Spreadsheets and Forms have something similar -> Properties Service, and more specifically the properties class.
Explanation
The extension mechanism for Google Documents, Spreadsheets and Forms is Google Apps Script. See the following guide: Extending Google Docs