You may add something to Erply, and wonder why it is not transferred to Magento (like it was with me here: Create new filter for category). In this case try to look in the table
ikff_erply_link
There is no documentation about that process of mapping (the WiKi info by Kostja is missing). Some clues of how it is used can be found here:
app\code\local\Magforge\Erply\Model\Processor\Local\Product.php,
_initProductAttributes(),
_initProductImages(), etc.