When categorising a transaction as a business income/expense, HMRC would require a copy of the receipt or invoice. It’s already possible to add those as an attachment to a transaction so the accountant would know how to categorise the transaction in their software. It’s also been mentioned as a possible feature (see below) that Monzo could potentially read the attachment using OCR text recognition technology to determine what that purchase was for and what category it fits into and from there how much VAT to calculate for that item.
How intelligent that would be and how well it’d work is another matter. It’d probably reduce the amount of input time but I’d stress the need for a human part of the process somewhere. The various technologies required aren’t quite advanced enough yet to be able to go completely autonomous.