<div class="seo-box expandable">
<?$APPLICATION->ShowViewContent('sotbit_seometa_bottom_desc');//вывод нижнего описания?>
<?echo $arSection["~UF_CATALOG_HTML"];?>
</div>
<?php
if(strlen($APPLICATION->ShowViewContent('sotbit_seometa_bottom_desc'))==0)
{
?>
<div class="seo-box expandable">
<?php echo $APPLICATION->ShowViewContent('sotbit_seometa_bottom_desc');//вывод нижнего описания
?>
<?php echo $arSection["~UF_CATALOG_HTML"];?>
</div>
<?php
}
?>
strlen($APPLICATION->ShowViewContent('sotbit_seometa_bottom_desc'))==0
empty($APPLICATION->ShowViewContent('sotbit_seometa_bottom_desc'))
<?=!empty($APPLICATION->ShowViewContent('sotbit_seometa_bottom_desc'))?$APPLICATION->ShowViewContent('sotbit_seometa_bottom_desc'):$arSection["~UF_CATALOG_HTML"];?>
$APPLICATION->ShowViewContent('sotbit_seometa_bottom_desc')
существует и не равна 0 и не false, то вынести$APPLICATION->ShowViewContent('sotbit_seometa_bottom_desc')
иначе вывести$arSection["~UF_CATALOG_HTML"]