Do you mean
a. "What should I store in the field?" or
b. "How should I set up my Mail Merge Main Document to do the merge? or
perhaps
c.  both?

If it's (a), I would put a 1 in the field if I wanted to print the letter
and either nothing or 0 if I didn't want to print the letter.

If it's (b), although it's possible to do something like

{ IF "{ MERGEFIELD letter_a }" = "1" "all the text and fields you want for
letter A"
}{ IF "{ MERGEFIELD letter_b }" = "1" "all the text and fields you want for
letter B"
}{ IF "{ MERGEFIELD letter_c }" = "1" "all the text and fields you want for
letter C"
}

and variations on that theme, I think it's generally better to do one merge
for each letter type and use the Mail Merge Selection options to select
letter_a Equals 1 for the first letter etc.

But I would experiment anyway, particularly with the selection stuff, as
WOrd has become more inclined to get that wrong in recent years.

Peter Jamieson