I quote:

"must be shown in all notas fiscais (tax invoices) issued by companies whose operations are subject to ICMS.
A nota fiscal is a very formal document and printers will only supply tax invoice orders if shown evidence that the buyer is a legitimate merchant registered for ICMS and authorized to order a new lot of them.

In theory, anyone carrying goods must be prepared to show the appropriate nota fiscal if so asked by a fiscal (tax inspector). In practice, you will see cars marked fiscalização (tax inspectors) parked on streets and roads and tax inspectors ordering trucks to stop for inspection. They will ask for the notas fiscais, which they will check against the mercadorias (goods).
"

depending on the context