Unfortunately, redacting an invoice are not to be done freely. There are some “industry” practices that your accounting will advise you on. Sometimes your Client’s will also have their own requirements because of internal control for instance. The rule of thumb is to always ask your accountant to help you issue an invoice in compliance with accounting laws and then send the draft to the Client together with the contract for approval. Once both documents are approved, your accountant or yourself may issue the invoice. Canceling an invoice is a long an fastidious process that should be prevented if possible!
Now concerning the subject of the VAT invoice, it has to do with the business activities that are registered under your Enterprise Registration Certificate. A company registered to provide investment advisory cannot provide marketing advisory, at least on the invoice. Issuing invoices for unauthorized or unregistered activities are an offense that is punished by law. Regarding foreign capital companies, you may as well fail the annual mandatory audit if your VAT invoices are not redacted properly. So please contact your advisor as this is not an easy matter!
See also What is a VAT/RED invoice ?