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.
[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.