Number used to uniquely identify a change master record.
All changes made with reference to a change number are controlled and logged by the change master record.
Numbers can be assigned both internally and externally.
Change Nos through Engineering Change Management (ECM) could be used for logging changes in Material Master, task lists , BOMs & documents etc. Its not mandatory that the changes should be through ECM only but in case your business scenarion requires complex data changes which you need to document then ECM is a must. Through ECM changed object's data is saved twice - its state before the change and state after the change . So you have complete history. Also in case you want some change to be effective from some future date , ECM would take care of that.