Well, "Smart tags" aren't really intended for that purpose, they aren't
fields, and I think it would be quite difficult to use them to do what
you're talking about.

Maybe you have Word 2007 and are referring to "Content Controls" - in which
case I suggest you have a look at Word Help for starters.

If you are using earlier versions of Word, you can use older field types but
a. typically, one location or field would "SET" the value
b. all the other fields would contain a copy of the value
c. you would typically have to use code to ensure that all the copies were
updated, unless you are using Word Online FOrms fields

In other words, prior to Word 2007 it's not easy to arrange that an update
to /any/ of the instances of the value results in changes to all the others.

There re several possible approaches, e.g.
a. use a SET, ASK or FILLIN field to set a bookmark, or bookmark a piece of
text, and use REF fields to make copies of the bookmark content
b. use a Document Property as the "master" version of the value, and use
DOCPROPERTY fields to insert copies of it
c. use VBA to create and maintain a "Document Variable" that contains the
value, and use DOCVARIABLE fields to insert copies of it

In some cases it might also make sense to use a STYLEREF field, or use Word
Online forms.

In Word 2007, head straight for content controls. In earlier versions, it
depends on what you need to achieve, whether you want to avoid VBA, etc.

Peter Jamieson