Typically following the three digits identifying the tyre's Section Width in millimeters is a two-digit number that identifies the tyre's profile or aspect ratio.
The 50 indicates that this tyre size's sidewall height (from rim to tread) is 50% of its section width. The measurement is the tyre's section height, and also referred to as the tyre's series, profile or aspect ratio. The higher the number the taller the sidewall, the lower the number the lower the sidewall. We know that this tyre size's section width is 225mm and that its section height is 50% of 225mm. By converting the 225mm to inches (225 / 25.4 = 8.86") and multiplying it by 50% (.50) we confirm that this tyre size results in a tyre section height of 4.43". If this tyre were a 225/70R16 size, our calculation would confirm that the size would result in a section height of 6.20", approximately a 1.8-inch taller sidewall.