diff --git a/htdocs/core/lib/functions.lib.php b/htdocs/core/lib/functions.lib.php
index 3a51575ef57..9311adb11ef 100644
--- a/htdocs/core/lib/functions.lib.php
+++ b/htdocs/core/lib/functions.lib.php
@@ -411,6 +411,9 @@ function dol_shutdown()
 
 /**
  * Return true if we are in a context of submitting the parameter $paramname from a POST of a form.
+ * Warning:
+ * For action=add, use:     $var = GETPOST('var');		// No GETPOSTISSET, so GETPOST always called and default value is retreived if not a form POST, and value of form is retreived if it is a form POST.
+ * For action=update, use:  $var = GETPOSTISSET('var') ? GETPOST('var') : $object->var;
  *
  * @param 	string	$paramname		Name or parameter to test
  * @return 	boolean					True if we have just submit a POST or GET request with the parameter provided (even if param is empty)