Any blood vessel may have a "blockage" due to internal clots/plaque
or external pressure.
The blockage can be graded and thus roughly estimated to be 25, 50, 50-75, more than 75%. Occasionally 60% is used as a cut off. Those numbers help to compare but also help to determine if blockage is significant enough to cause harm. Less than 50% says it is not significant. Test can monitor changes.