In BuddyPress, What is the use of is_default_option field in wp_bp_xprofile_fields table?
This is one of the things you won't find in the docs, but only in the source. The source code contains only one usage of this variable. This is in bp-xprofile-template.php line 493.
As you can see, it is used to pre-select an option in the case the user hasn't supplied a value yet.