When you create a client customfield, it's available in .tpl files in two different Smarty variables:

• an array like this:
   customfields => array (18)
      0 => array(2)
            id =>83
            value => "mycustomvalue"
  1 => array(2)
        id =>87
        value => "mysecondcustomvalue"
  
  etc.

• And a series of useless variables like:
      customfields1 = "mycustomvalue"
      customfields2 = "mysecondcustomvalue"

The first one does not allow any easy and compact way to get the value of a custom field, which can be found only through a loop:

{foreach from=$customfields item=field}
    {if $field.id == 17}
        {$field.value}
    {/if}
{/foreach}

but this is not efficient, and far from being error proof...

The second one instead is almost useless because there's no safe way to relate each smarty variable to a custom field; if you create a new custom field the order may change, and you'll have to edit all your custom tpl files...

An elegant solution is the following hook:

add_hook('ClientAreaPage', 1, function($vars) {    
    if (!empty($vars['clientsdetails']['customfields'])) {
        $customfieldsAssoc = [];
        foreach ($vars['clientsdetails']['customfields'] as $field) {
            $customfieldsAssoc[$field['id']] = $field['value'];
        }
        return ['customfieldsAssoc' => $customfieldsAssoc];

    }
});

This hook makes available the value of any client custom field in a template, using i.e. the form

{$customfieldsAssoc.17} 

(where "17" in the example is the id field in the tbl customfields)

Reference: https://whmcs.community/topic/348184-how-to-recover-client-customfields-values-in-smarty/