If the text you want to share is only a part of any of your sub-documents, you need to identify and label such parts.
o Select the relevant text.
o Go to Insert | Section... | Section.
o Give the "New section" a name.
o Click OK. You will see that sections need to be complete paragraphs or collections of paragraphs.
In the document which needs to inherit the text,
o Go to Insert | Section... | Section.
o Tick Link.
o Click "..." next to "File name".
o Browse to the sub-document.
o Where relevant, select the appropriate part under Section.
It's possible that you will need to save sub-documents before any changes in them will be reflected in the main document. And it's certainly the case that you will need to update the main document if it is already open in order that any changes are reflected there. To do this, go to Tools | Update > | Links (or Tools | Update > | Update All).
All this is necessary only if you later need to edit the sub-documents and wish to see changes reflected in the main document, of course. Otherwise, you can just copy an paste text between documents.
I trust this helps.
Brian Barker