What is the extra field added with a combo or list box

I posted this question once already but I think I forgot to specify a
discussion group, so here it is again, sorry if it's a duplicate.

Anyway... some of the time but not always, when I add an unbound combo or
list box control to a form, Access adds a corresponding column to the data
sheet portion of the form. Usually my reason for the control is to look up
and go to entered values. The extra field holds the value that is selected
in the control, but doesn't seem to do anything else. If I hide the column
the control will no longer do the search. What is this extra column and what
can I do differently to get rid of it? Any help would really be appreciated,
thanks!