More detail
Think of the CU number as the fingerprint of an individual invoice. It makes each tax invoice traceable rather than just a slip of paper.
Because the system generates it automatically, the practical thing for a shop is simply that it appears on compliant receipts; you do not have to manage or assign these numbers yourself.
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Related questions
Do I have to generate the CU number?
No. The eTIMS system assigns it automatically when the invoice is issued. You do not create or manage it.
Why does the CU number matter?
It uniquely identifies the invoice so it can be verified against KRA records, which is part of what makes the receipt a valid tax invoice.
Veira gives Kenyan shops one calm app for selling, M-Pesa, KRA eTIMS and stock, with a free terminal. Book a demo and see it set up for your trade.