In `multipath.conf` file, why wwid begin with 3?

Issue

• Would like to know what is difference for following wwid begin at 3

  multipath {
              wwid                    600601607ef03400a80573e4c75ee543
              alias                   abc
      }
      multipath {
              wwid                    3600601607ef03400a80573e4c75ee543
              alias                   abc1
      }