The series volume, which is the ongoing story. A change in series volume can refer to a reboot or relaunch of the series This field is used to differentiate multiple series which share the same series title and are about the same character or characters. Volume numbers represent the chronological sequence of issue, and are usually found in the indicia.
For example, Marvel published Daredevil consecutively from 1964 through to 1998 without renumbering or rebooting. Issues #1-#380 printed over this time are Volume 1 in the series. Volume 2 of Daredevil started in 1998, after the series was cancelled and started over.
This is as opposed to the Collected Volume, which refers to a book or volume containing several issues of the same series. E.g. In 2011, Daredevil was up to Volume 3, but Marvel also released the first six issues as Daredevil Volume 1, and the next 6 as Volume 2. They then reprinted the same series in hardback of 10 issues, which were also termed Volume 1.