BR-FXEXT-11
Each "ID of parent line" (BT-X-304) must refer to an existing "ID of invoice item" (BT-126).
error: the document is invalid
What the rule checks
As the rule set's publisher wrote it. The rule runs on every element the context matches, and fails where the test is false; the validator reports the line of that element.
Context
//ram:IncludedSupplyChainTradeLineItem [normalize-space(ram:AssociatedDocumentLineDocument/ram:ParentLineID) != '']
Test
some $p in //ram:IncludedSupplyChainTradeLineItem/ram:AssociatedDocumentLineDocument/ram:LineID satisfies normalize-space($p) = normalize-space(ram:AssociatedDocumentLineDocument/ram:ParentLineID)
Checked with France Factur-X EXTENDED, Factur-X / ZUGFeRD EXTENDED