This addition to the operating system in no way changed the logical structure of the file system.
Formerly, HFS Plus volumes were embedded inside an HFS standard filesystem.
This was phased out by the Tiger transition to Intel Macs, where the HFS Plus filesystem was not embedded inside a wrapper.
These features were accessible through the GUI, using the Disk Utility application, in Mac OS X Server, but only accessible through the command line in the standard desktop client.
With Mac OS X v10.3, all HFS Plus volumes on all Macs were set to be journaled by default.
These sectors are then grouped together into allocation blocks which can contain one or more sectors; the number of allocation blocks depends on the total size of the volume.