An invoice, invoice or tabulator is a commercial document issued by a seller to a buyer in connection with a sales transaction and indicates the products, quantities and agreed prices for the products or services that the seller has provided to the buyer.
The payment terms are generally indicated on the invoice. These can specify that the buyer has a maximum number of days to pay, and sometimes a discount is offered if paid before the due date. The buyer may have already paid for the products or services listed on the invoice. To avoid confusion and unnecessary subsequent communication from buyer to seller, some sellers clearly indicate in large and / or capital letters on an invoice whether it has already been paid.
From the seller’s point of view, an invoice is a bill of sale. From the buyer’s point of view, an invoice is a purchase invoice. The document indicates the buyer and the seller, but the term invoice indicates that the money is owed or owed.
A typical invoice can contain:  
The word invoice (or tax invoice);
A unique reference number (in case it matches the invoice);
Date of invoice;
Tax amounts, if relevant (for example, GST or VAT);
Name and contact details of the seller;
Seller’s business registration or tax details, if relevant, e.g. Eg ABN for Australian companies or VAT number for EU companies;
Buyer’s name and contact details;
Date of shipment or delivery of the goods or services;
Purchase order number (or similar tracking numbers requested by the buyer to be mentioned on the invoice);
Description of the product (s);
Unit price (s) of the product (s), if relevant;
Total amount collected in currency (symbol or abbreviation);
Payment terms (including one or more acceptable payment methods, the expected payment date, and details of late fees, that is, payments made after that date);
In countries where wire transfer is the preferred method of paying debts, the printed invoice will contain the creditor’s bank account number and usually a reference code that will be transmitted along with the transaction identifying the payer.
The US Defense Logistics Agency requires an employer identification number on invoices. 
The European Union requires a VAT (value added tax) identification number for official VAT invoices, which all companies with a VAT number must issue to their customers. In the UK, this number can be omitted on invoices if the words “this is not a VAT invoice” are present on the invoice. Such an invoice is called a pro forma invoice and is not a suitable substitute for a full VAT invoice for VAT registered customers. 
In Canada, the GST registration number must be provided for all supplies over $ 30 made by a registered supplier to claim incoming tax credits. 
The ECE Trade Committee also provides recommendations on invoices used in international trade, which implies a more detailed description of the logistical aspect of the goods and therefore may be suitable for international logistics and customs procedures. 
Definition of EU VAT union invoice
Within the European Union Value Added Tax Directive , Article 226 provides a concise definition of invoices within the Member States of the European Union.
Without prejudice to the special provisions established in this Directive, only the following data are required for VAT purposes in invoices issued in accordance with articles 220 and 221:
the date of issue;
a progressive number, based on one or more series, that uniquely identifies the invoice;
the identification number for VAT purposes referred to in article 214 with which the taxable person has delivered the goods or services;
the customer’s VAT identification number, mentioned in article 214, from which the customer has received a delivery of goods or services for which he is obliged to pay VAT, or has received a delivery of goods mentioned in article 138;
the name and full address of the taxable person and the customer;
the quantity and nature of the goods supplied or the extent and nature of the services rendered;
the date on which the supply of goods or services was made or ended or the date on which the advance payment referred to in points 4 and 5 of article 220 was made, insofar as this date can be determined and is different from the date of issuance of the invoice;