Custom Script example in the documentation - Syntax of the abstract looks incorrect

I am trying to understand Custom Scripts and doing so started to study some examples given in the Documentation

i.e. this one

Custom Scripts / Fetch Values From Master

Custom Script for Fetching Values From Master

To pull a value of a link on selection, use the add_fetch method.

add_fetch(link_fieldname, source_fieldname, target_fieldname)


Let’ say you have created a Custom Field VAT ID ( vat_id ) in Customer and Sales Invoice and want to make sure that this value gets updated every time you select a Customer in a Sales Invoice.

To configure this, in the Sales Invoice Custom Script, you can add this line:


now the abstract add_fetch(link_fieldname, source_fieldname, target_fieldname) looks not quite correct to me.

comparing it to the concrete example

abstract: add_fetch(link_fieldname, source_fieldname, target_fieldname)
concrete: cur_frm.add_fetch('customer','vat_id','vat_id')

looks to me as if the abstract should be
add_fetch(doctype, source_fieldname, target_fieldname)

customer is not a link_fieldname, but the doctype you are fetching the data from in this example. Isn’t that so?