When importing files from KuCoin's "Billing History", all files with margin trades are rejected (both the Order-Splitting and not) with message:
This file is not needed. You can import your other order file instead, which will contain the same data without the 'Order-Splitting'
Workaround
I need to edit the non-order-splitting file and modify one column name (KuCoin exports it with a typo) - I need to replace
Fee Currency,
with
Fee Currency
(note the comma)