An invoice or bill is a commercial document issued by a seller to the buyer, indicating the products, quantities, and agreed prices for products or services the seller has provided the buyer. An invoice indicates the buyer must pay the seller, according to the payment terms.
In the rental industry, an invoice must include a specific reference to the duration of the time being billed, so rather than quantity, price and discount the invoicing amount is based on quantity, price, discount and duration. Generally speaking each line of a rental invoice will refer to the actual hours, days, weeks, months etc being billed.
From the point of view of a seller, an invoice is a sales invoice. From the point of view of a buyer, an invoice is a purchase invoice. The document indicates the buyer and seller, but the term invoice indicates money is owed or owing. In English, the context of the term invoice is usually used to clarify its meaning, such as "We sent them an invoice" (they owe us money) or "We received an invoice from them" (we owe them money).