ok, then i have to explain it a little bit more detailed.
I had to register field birthday in session.php to make it an "default" field cause of the following reason:
Usually you are right vano,
cause in db_field_definiton.php, when i register a field,
the additonal_sql functions is used and the changes are in effect.
But for this mod i can't use the aditional_field as it usually was made for,
cause from member.php/register.php there aren't submit just the birthday field.
There are submitted 3 values (day/month/year) to use in dropdown options when entering the user's birthday.
So i needed to update the additional_sql manually,
after the additional fields are checked if they are filled or not.
cause no direct HTTP POST VAR for birthday is submitted.
And thats the reason why i did it like this way,
so there is no need to change the whole code, if changing the db field name.
Second reason was the integration in my board, and hold the field name flexibile for future modifications,
if i had to use a other board software and change the field name again.
Of course, you are right vano,
the second possibilty would have been just to change per phpmyadmin the db fieldname,
but i couldn't do that, because of the board integration.
(Otherwise i had to change the board code for db field name modification again)
So i think that was the best way to do it.
I know it's almost very unique to do it like this.
P.S: the additional_field registration is only needed to display users birthday value in admin control,
and register the vars automatically.