The lower the number, the warmer the glow plug. Warmer glow plugs are used for lower performance engines, fuel with low nitro content or difficult running and tuning conditions. Colder glow plugs are for high performance engines, higher nitro content fuel and fine tuned engines which are running a leaner fuel mixture. Many high performance car engines use turbo type plugs. Make sure you choose the right glow plug for your engine.
