['section', 'custom_section'],
After further analyzing the debug info and seeing that it has parameter
context_title => courseprefix-coursenumber-sectionnumber-semester
this actually worked for me for setting section number automatically from Blackboard (in authen_LTI.conf)
$LTI_modify_user = sub {
# The self object from LTIAdvanced.pm
my $self = shift;
# The user object to be modified
my $user = shift;
my $r = $self->{r};
my @course_id = split /-/, $r->param("context_title");
$user->{"section"} = $course_id[2];
};