Shell icon overlays are icons that Microsoft Windows can display on top of folder icons. A list of icon overlays is specified in the Windows Registry in the location mentioned further in this article. In the current implementation there are 4 bits in a structure that define the overlay index providing for a maximum of 15 overlays, 4 of which are reserved by the operating system.^[1] Therefore, even though more than 11 icon overlay handlers can be registered, only the first 11 icon overlays supplied by icon overlay handlers are effectively displayed. The remaining icon overlay handlers are not used.^[2]

Many applications such as versioning software like TortoiseSVN and cloud storage synchronization software like Nextcloud, Dropbox, and OneDrive add their own icon overlay handlers to the Registry upon installation. Below is a table of shell icon overlay identifiers by software.

All shell icon overlay identifiers are located in the Windows Registry.

Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\ShellIconOverlayIdentifiers

Category	Software	Registry Name	Icon	Description (Symbol)	Icon Version (Date)	Source
Cloud Storage Services	Adobe Creative Cloud	AccExtIco1	[3]	Synced file or folder; finished sync.
		AccExtIco2	[3]	In process of syncing to ACC servers. (Symbol: Horizontal blue arrows)
		AccExtIco3		unknown (Symbol: unknown)
	Dropbox	DropboxExt01	[4][5]	Synced file or folder; finished sync. Downloaded local content on your hard drive.[6] (Symbol: Solid Green check ✓)	v73.4.118(May 24, 2019)	See inside each cell.
		DropboxExt02	[4][5]	Sync in progress; in process of syncing to Dropbox servers.[6] (Symbol: Blue w/ rotating arrow)
		DropboxExt03	[4]	[4](Year: 2019): Sync not happening; something wrong due to invalid file names, permission error, rapid multiple edits, or out of storage space.[6] (Symbol: Red X) OR [5] (Year: 2014)
		DropboxExt04	[4]	[4](Year: 2019): A file or folder is not syncing when using the Selective-sync feature (meaning you have opted not to sync this file or folder on your computer). In general, selectively synced files or folders will not appear in Dropbox Folder on your computer. However, if you create two files or folders with the same name, the gray icon will appear, indicating that one of the files is not being synced.[6] (Symbol: Gray minus) OR [5](Year: 2014)
		DropboxExt05	[4]	[4](Year: 2019): Locked: Synced!; View-only access to file or folder. Read-only, cannot edit or share with others. Request edit access with owner.[6] (Symbol: Green check ✓ w/ Lockpad) OR [5](Year: 2014)
		DropboxExt06	[4]	[4] (Year: 2019): Locked: Sync in progress; View-only access to file or folder. Read-only, cannot edit or share with others. Request edit access with owner.[6] (Symbol: Blue w/ rotating arrows and w/ Lockpad) OR [5](Year: 2014)
		DropboxExt07	[4]	[4](Year: 2019): SmartSync feature, Online-Only content. These appear in your Dropbox Folder on your computer, but they are not fully downloaded. Placeholder icons.[6] (Symbol: Grey w/ white cloud shape) OR [5](Year: 2014)
		DropboxExt08	[4]	[4](Year: 2019): SmartSync feature, Online-Only content. These appear in your Dropbox Folder on your computer, but they are not fully downloaded. Placeholder icons.View-only access to file or folder. Read-only, cannot edit or share with others. Request edit access with owner.[6] (Symbol: Grey w/ white cloud shape and w/ Lockpad) OR [5](Year: 2014)
		DropboxExt09	[4]	Mixed State: contains both local and online-only files.[6] (Symbol: White and Green check ✓)
		DropboxExt10	[4]	Locked: Read-only; Mixed State: contains both local and online-only files.[6] (Symbol: White and Green check ✓ w/ Lockpad)
	OneDrive Personal	OneDrive1		Cannot Sync (i.e. ErrorOverlayHandler Class) (Symbol:Red X)	v19.070 (May 13, 2019)	1,2,3,4
		OneDrive2		Shared with others (i.e. [SharedOverlayHandler Class) (Symbol:Green + People)
		OneDrive3		Online-only files do not take up space on your device, file is only available online, but can be downloaded with internet. (i.e. UpToDateCloudOverlayHandler Class) (Symbol: Blue cloud)
		OneDrive4		Files is "Always-keep-on device". 'Always available files' download to device and take up space, always available even without internet. (i.e. UpToDatePinnedOverlayHandler Class) (Symbol: Green ✓ w/ solid green background)
		OneDrive5		Sync in progress (i.e. SyncingOverlayHandler Class) (Symbol: Rotating blue arrows)
		OneDrive6		Locked (read only); Has settings preventing it from syncing. (i.e. [ReadOnlyOverlayHandler Class) (Symbol: Green Lockpad)
		OneDrive7		When you open an online-only file, it downloads to your device and becomes locally available file, even without internet. (i.e. UpToDateUnpinnedOverlayHandler Class) (Symbol: Green ✓ w/ white background)
	OneDrive for Business (formerly SkyDrive)	SkyDrivePro1 (ErrorConflict)		Error Conflict (Symbol: Red X)	v19.070 (May 13, 2019)	1
		SkyDrivePro2 (SyncInProgress)		Sync in Progress (Symbol: Blue rotating arrow)
		SkyDrivePro3 (InSync)		In Sync (Symbol: Solid Green check ✓)
	NextCloud	OCError		Sync Error or Blacklisted file (Symbol: Red X)	v2.5(Apr 11, 2019)	1
		OCOK		File is Synced w/ server (Symbol: Solid Green check ✓)
		OCOKShared		Shared (Symbol: Green share icon)
		OCSync		Sync in progress (Symbol: Blue w/ rotating arrow)
		OCWarning		Sync Error or Warning (Symbol: Yellow exclamation !)
	Google Backup & Sync (formerly Google Drive)	GoogleDriveBlacklisted		Not able to Sync (Symbol: Red X)	v3.43.4275 (Oct 15, 2018)	1
		GoogleDriveSynced		Synced (Symbol: Green check ✓)
		GoogleDriveSyncing		Sync in progress (Symbol: Rotating blue arrows)
	odrive	0drive.Active		Sync in progress (Symbol: Fuchsia w/ rotating white arrows)	v6627 (Jul 01, 2020)
		0drive.Synced		Synced (Symbol: Light blue w/ white check ✓)
Source Code Management System clients	TortoiseSVN, TortoiseGit and TortoiseHg	Tortoise1Normal		Normal (Symbol: Solid Green check ✓)	v1.12.0 (Apr 17, 2019)	1,2
		Tortoise2Modified		Modified (Symbol: Red !)
		Tortoise3Conflict		Conflict (Symbol: Yellow triangle !)
		Tortoise4Locked		Locked (Symbol: Yellow Lockpad)
		Tortoise5ReadOnly		Read only (Symbol: Grey check ✓)
		Tortoise6Deleted		Deleted (Symbol: Red X)
		Tortoise7Added		Added (Symbol: Blue plus +)
		Tortoise8Ignored		Ignored (Symbol: Grey minus)
		Tortoise9Unversioned		Unversioned (Symbol: Blue question mark ?)
NTFS link Manage	Link Shell Extension	HardLinkMenu		Context Menu	v3.9.3.1 (Nov 23, 2019)
		IconOverlayHardLink	/	Hardlink (Symbol: Red arrow)
		IconOverlayJunction	/	Junction (Symbol: Metal link)
		IconOverlaySymbolicLink	/	Symbolic Link (Symbol: Green arrow)