Many types of electrical wire can be used outside, including 18 AWG wire. TFFN is Thermoplastic Flexible Attachment Nylon. It's most commonly used for "fixture" and lighting. It has a nylon coating that is applied to the PVC insulation.
TFFN is an industry term for electrical wire. It's also called fixture wire and building wire. Electric wire as well as many other types and styles of heat shrink tubing and portable cord can be found at baboombaboom.com/18-gauge.
This electric wire can be used with multiple wires. You have two options: tray cable (PLTC) or power limited tray cable. PLTC can be used at 300 volts, while standard tray cable can handle 600 volts.
However, if you require it in 18 AWG, you will need the power limited version. PLTC cables are approved for outdoor use and conduit usage, just like TFFN.
An additional jacket holds multiple wires together so they can be run all at once, rather than separately. This jacket saves time and makes it easier to install multiple wires at once.
Both types of electric wire have bare copper strands, which allow for easy installation in conduit, homes, and buildings. When installed, less flexible wires will hold their form better.
Talk to your cable and wire supplier about which type of wire will be best suited for your application. You might find a cheaper 18 AWG version that suits your application at a lower price.