On the Adminorders.php page, one of the fields under "Additional Details" is "Affiliate" - how can I change that word to something else? I looked in the languagefile_en.php - but there is nothing that shows simply the word "Affiliate".

There is a separate file for admin text so that would be in...
languageadmin.php
...and is the following...
$yyAffili='Affiliate';

Vince

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