First, drop the term sub document. It will get you misleading results in web searches. See Master Documents "Feature".

I do this through fields. It all depends on how you want to store the information.
One method would be to store the basic information to be inserted in either a Word table or an Excel spreadsheet and use the Mail Merge feature in Word to pull the information into the other documents in merge fields. Although mail merge was originally designed for mass mailings, it works well creating single documents with information stored elsewhere. I have more than 100 document templates that use this method with one data source.

Another method if the information does not change a lot is to use AutoText stored in a Global Template. You would then use AutoText fields to insert the information in the documents.

If you are using legacy formfields to gather the information, they can have bookmark names. You could use IncludeText fields to bring in the text. This method might be easier for the user but would be a pain to set up and maintain.