Your post is a little elliptical--but some pointers.

You could use a LISTNUM or a SEQ field series.

Here's an excellent explanation of SEQ fields:
http://groups.google.com/group/microsoft.public.word.docmanagement/browse_fr
m/thread/2648a63b2030cc69/7c922df43378a0fe

(Line wrap may break URL)

See Help on SEQ field for the switch that would make it letters rather than
numbers.

I'm not very familiar with ListNum fields, but I think they work similarly
to SEQ fields, except that they live-update, while with SEQ fields you have
to select all and hit F9 for the numbers to update.  Again, investigate
Help.

The field code might be set up as an AutoText, if you choose.

Once you have set up the list, you can use cross-references to get the
proper letter into the text as you refer to it.

If that doesn't point you in the right direction, you might want to explain
what you are talking about and give a fuller explanation of the situation
you are dealing with.