[Victor D]

productid is an numeric internal xcart value. It is used in database as a primary key so it is unique. If you browse to any product you will see URL like this
http://yourstore/product.php?productid=14334
14334 is a productid

you can also use SKU:

{if $main eq "product"}<script src="http://www.yakkingheads.com/sample
{$product.productcode|escape}.js">  </script>{/if}

or even product title:

{if $main eq "product"}<script src="http://www.yakkingheads.com/sample
{$product.producttitle|escape}.js">  </script>{/if} 

but in last two cases you can get an invalid URL for your script as either title or SKU may contains characters that are not allowed to use in URI