The pulley timing belt connects the motor cog with the wheel cog, transferring drive from the motor to the wheel.
Your belt needs to have the same width (mm) as your pulleys and the same HTD rating. This gives the most contact with the pulleys as possible helping your boards efficiency.
Specs
Belt Pitch: 5mm
Belt Width: 12mm
Belt Length: 250mm/330mm/360mm
250mm for PU wheel
330mm and 360mm for AT wheel