I replied to my own post last night (well, very early this morning really), but I must have accidently hit Preview instead of Submit or something because I don't see my reply now. The issue was as I suspected that $f_products is a Smarty variable so it can't be called as is from a PHP file.
I have now replaced $f-products with
$smarty->_tpl_vars['f_products'] in my welcome_tabs.php file and everything is working fine. The tab appears with my Featured products when I have Featured products selected, and no tab appears when I deactivate or delete the Featured products.
I call my welcome_tabs.php file - include $xcart_dir . '/include/welcome_tabs.php'; - from home.php after the include './featured_products.php'; line. Then I replaced the following code in welcome.tpl
Code:
{if $active_modules.Bestsellers and $config.Bestsellers.bestsellers_menu ne "Y"}
{include file="modules/Bestsellers/bestsellers.tpl"}<br />
{/if}
{include file="customer/main/featured.tpl"}
with
Code:
{if $welcome_tabs}
{include file="customer/main/ui_tabs.tpl" prefix="welcome-tabs-" mode="inline" tabs=$welcome_tabs}
{/if}
I also plan to modify the featured.tpl and bestsellers.tpl templates for appearance.
I haven't looked at it closely yet, but I plan to modify subcategories.tpl as I did welcome.tpl so I can have the tabbed appearance on subcategory pages, too.