The item guid is randomly generated on the client and is used to identify each lockwise entry (login + pw) that is stored. It applies only to the addon and is not shared across other platforms.
Bug 1555821 Comment 3 Edit History
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EDIT: I had this wrong actually . The item guid is randomly generated and used to identify each lockwise entry (login + pw) that is stored. These guids ARE CONSTANT ACROSS PLATFORMS, and could appear in lockwise ios/android telemetry, if the items are accessed there. There is no other "master list" where these guids are avaliable to mozilla, however.