Einvoice Generation

Hello friends, want to know the rules regarding Einvoice, one of our client has raised the sale invoice on 23/10/2023 but generated the Einvoice on 25/10/2023, whether the Einvoice generated by my client is valid?. also want to know the criteria and legality of the same. who has to generate the Einvoice and the person who is eligible for not generating the einvoice.