Example of how to change the calendar available spaces to use the first ticket within an event if the ticket name contains 'Participant'

example.php

<?php

add_filter('FHEE__EE_Calendar__tooltip_datetime_available_spaces', 'tw_calendar_datetime_available_spaces', 10, 2 );
function tw_calendar_datetime_available_spaces( $default_value, $datetime ) {
	
	$query_params = array(
		array(
			'TKT_deleted'    => false,
		),
		'order_by' => 'TKT_order',
		'order' => 'ASC'
	);
	
	$first_ticket = $datetime->get_first_related('Ticket', $query_params );
	
	if( $first_ticket instanceof EE_Ticket && strpos($first_ticket->name(), 'Participant' ) !== false ) {
		return $first_ticket->qty('saleable');	
	}
	
	return $default_value;
}