What is the proper storage position for refrigerant cylinders?

The concept behind this question

Once refrigerant is in a cylinder and leaving the site, it is a hazardous material shipment and Department of Transportation rules apply on top of EPA's. Recovered refrigerant goes in a DOT-approved recovery cylinder — grey body with a yellow top — never in a disposable cylinder the refrigerant did not come in. Cylinders must be labelled with their contents, kept within the 80% fill limit, and accompanied by shipping papers. Mixing different refrigerants in one cylinder makes the contents unreclaimable and turns a recoverable asset into disposal cost.