The HMRC systems will reject submissions that do not pass validation tests - for example not having mandatory fields populated, or values that appear to contradict each other. Therefore the ABC application will insist that all errors have been fixed before submission.  However, since the ABC application performs all the calculations that HMRC will do, you are able to view the total tax you owe based on the data you have entered at any point, even if you have validation errors outstanding.

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