What is a primary characteristic of LPG that must be documented during sale?

The primary characteristic of LPG (liquefied petroleum gas) that must be documented during sale is the price per gallon. This is essential because it ensures transparency in transactions, allowing consumers to understand the cost they are paying for the gas they are purchasing. The price per gallon provides a basis for comparison among suppliers and helps to establish fair market practices. Additionally, documenting the price is crucial for regulatory compliance and consumer protection, as it can prevent price gouging and disputes over costs.

While other characteristics such as total weight, vapor pressure, and type of gas are important for different purposes—like ensuring the product's quality and safety—they are not as critical for the actual transaction documentation. The price per gallon is directly tied to consumer rights and market competition, making it the key focus during the sale process.