windows 8 - In chkdsk, what is an online scan?

Running Windows 8.0 Pro, chkdsk has a /scan option that Runs a online scan on the volume (sic). What is an online scan?

My guess is that it runs it while letting you use the drive, which is good if it's your c: drive.

Here's the full options:

CHKDSK [volume[[path]filename]]] [/F] [/V] [/R] [/X] [/I] [/C] [/L[:size]] [/B]
[/scan] [/spotfix]


  volume              Specifies the drive letter (followed by a colon),
                      mount point, or volume name.
  filename            FAT/FAT32 only: Specifies the files to check for
                      fragmentation.
  /F                  Fixes errors on the disk.
  /V                  On FAT/FAT32: Displays the full path and name of every
                      file on the disk.
                      On NTFS: Displays cleanup messages if any.
  /R                  Locates bad sectors and recovers readable information
                      (implies /F, when /scan not specified).
  /L:size             NTFS only:  Changes the log file size to the specified
                      number of kilobytes.  If size is not specified, displays
                      current size.
  /X                  Forces the volume to dismount first if necessary.
                      All opened handles to the volume would then be invalid
                      (implies /F).
  /I                  NTFS only: Performs a less vigorous check of index
                      entries.
  /C                  NTFS only: Skips checking of cycles within the folder
                      structure.
  /B                  NTFS only: Re-evaluates bad clusters on the volume
                      (implies /R)
  /scan               NTFS only: Runs a online scan on the volume
  /forceofflinefix    NTFS only: (Must be used with "/scan")
                      Bypass all online repair; all defects found
                      are queued for offline repair (i.e. "chkdsk /spotfix").
  /perf               NTFS only: (Must be used with "/scan")
                      Uses more system resources to complete a scan as fast as
                      possible. This may have a negative performance impact on
                      other tasks running on the system.
  /spotfix            NTFS only: Runs spot fixing on the volume
  /sdcleanup          NTFS only: Garbage collect unneeded security descriptor
                      data (implies /F).
  /offlinescanandfix  Runs an offline scan and fix on the volume.

The /I or /C switch reduces the amount of time required to run Chkdsk by
skipping certain checks of the volume.

Answer

An offline scan is done by dismounting the volume to fix file system corruption and bring the volume back to a healthy state. Generally it is only possible with non-system volumes, and system volumes (with the dirty bit set) are scanned during boot-up before the OS is up and running completely.

Self-healing is a feature built into NTFS that fixes certain classes of corruptions encountered during normal operation, and can make these fixes while the volume is still online. If all issues that are detected are self-healed online then obviously there is no need for an offline repair. In Windows 8 the number of issues that can be handled online has been increased and hence the need for running an offline scan using chkdsk is further reduced. All this is part of the new NTFS health model:

You can also run an on-demand online scan with chkdsk /scan to identify and spot-fix file system corruption, although for major problems a full offline scan is still required.

_{Source: Redesigning chkdsk and the new NTFS health model}