This Filter can be used to add custom access rule type in admin side access rule grid only but not add rules for this custom type. So use this filter with other filters like arm_prepare_custom_rule_data, arm_before_update_custom_access_rules to add rules for newly added custom type for access rules.

Hook Type: Filter
add_filter( 'arm_custom_rule_types', 'your_function');
function your_function($rule_types = array()) {
    //$rule_types Custom Access Rule Types.
    $rule_types['custom_rule'] = 'Custom Rules';
    return $rule_types;
Custom Access Rule Types