Also the price can be printed in the email as #_TOTALPRICE{xx} where xx is the number of the multi options. How can I also print the Price Categories descriptions?

Thanks Franky 🙂

This is the code you mean, and how do I reach the individual price categories with the function?
$_POST[‘bookings’][$key][‘bookedSeats’]{1}
$_POST[‘bookings’][$key][‘bookedSeats’]{2}
… ?

add_filter('eme_eval_multibooking_form_post_filter','do_my_stuff');
function do_my_stuff($events) {
    // it is recommended to add a hidden field in your form, and we check on that
    if (!isset($_POST['my_hidden_field'])) return;

    // as an example, let's just count the number of seats booked in total, to be sure
    $total_booked=0;
    foreach ($_POST['bookings'] as $key=>$val) {
        // format posted: $_POST['bookings'][$event_id]['bookedSeats']
        $total_booked+=$_POST['bookings'][$key]['bookedSeats'];
    }

    // do something for all events being registered for
    // foreach ($events as $event) {
    // .... (see the example for eme_eval_booking_post_filter)
    // }
    if ($total_booked>0) {
       // eval is success
       return array(0=>1,1=>'');
    } else {
       // bad form entry
       return array(0=>0,1=>'Not correct seats registered');
    }
}

Thanks Frank, this is the code and it works like a charm. 🙂

add_filter('eme_eval_booking_form_post_filter','check_accommodation_option');
function check_accommodation_option($event) {
    $event_id=$event['event_id'];
    $check_option_ok=0;

    // only do a specific event
    if ($event_id != 5) return;

    // only allow one booking of type Acommodation
    if($_POST['bookings'][$event_id]['bookedSeats3']==1 && $_POST['bookings'][$event_id]['bookedSeats4']==1  ){
      $check_option_ok=1;
    }

    if (!$check_option_ok) {
       // eval is success
       return array(0=>1,1=>'');
    } else {
       // bad form entry
       return array(0=>0,1=>'Please check only one Accommodation option');
    }
}

Thanks for the added info Franky. I had the event id 5 so all good 🙂
So appreciate your help!