The distinction matters because petitions and interim orders don't end a marriage; final orders do. UKVI's published guidance asks for evidence that the previous marriage has ended, which means evidence that the divorce is final under the law of the issuing country.
In England and Wales, this is the Final Order (formerly Decree Absolute). The Decree Nisi or Conditional Order is an interim step and is not, on its own, sufficient evidence that the marriage has ended. Files that submit only a Decree Nisi get an RFI.
In Pakistan and Bangladesh, the talaq process under Muslim Family Laws Ordinance 1961 requires the talaq pronouncement, notification to the Union Council, an arbitration period (usually 90 days), and a Union Council certificate confirming the talaq has taken effect. The certificate is the document UKVI needs. The talaq nama alone, without the certificate, is the equivalent of a petition: the process has started but not concluded.
In most civil-law European countries (Spain, France, Italy, Germany), the court issues a final judgment of divorce (sentencia firme de divorcio, jugement de divorce devenu définitif, sentenza di divorzio passata in giudicato). The 'passed in res judicata' or 'definitive' wording is what UKVI looks for, distinguishing the final judgment from the initial decision that may have been subject to appeal.
In the US, the document is the Divorce Decree from the relevant state court, which serves as the final order.