A specimen banknote is printed to as a way of checking to see whether or not the design is suitable for putting into full production as a currency issue.

Sometimes, specimen banknotes are printed for distribution to central banks to aid in the recognition of banknote

Banknote

A banknote is a kind of negotiable instrument, a promissory note made by a bank payable to the bearer on demand, used as money, and in many jurisdictions is legal tender. In addition to coins, banknotes make up the cash or bearer forms of all modern fiat money...

s from a country other than their own.

To avoid its use as a legal tender note the banknote is perforated or punched, given a serial number of 0.9999999999 (all nines) or 0000000000 (all zeroes) or is overprinted with the word Specimen or Cancelled in any language.