You discover that Windows Shell doesn't display some icon overlays that are provided by registered icon overlay handlers. This behavior affects applications that register an icon overlay handler to provide extra information about a file or a folder.
The number of icon overlay handlers that the system can support is limited by the amount of available space for icon overlays in the system image list. There are currently 15 slots allotted for icon overlays, 4 of which are reserved by the system. When there are more than 11 icon overlay handlers registered in the system, only the first 11 icon overlays that are provided by icon overlay handlers are added to the system image list. The remaining icon overlay handlers are not used.
Software developers should not rely solely on icon overlay handlers to provide extra information about files or folders. Extra information about a file or folder should also be accessible to users through other means, such as through the Windows Property System.
An icon overlay is a small image in the lower-left corner of an icon that represents a Shell object. An icon overlay is typically added to an object's icon to provide extra information. For example, a commonly used icon overlay is the small arrow that indicates that the icon represents a link instead of the actual file or folder. In addition to the standard icon overlays that are provided by the system, software developers can provide custom icon overlays by implementing and registering an icon overlay handler.
Microsoft OneDrive and OneDrive for Business register multiple icon overlay handlers to indicate the synchronization and share states of the files that are managed by those applications. Because OneDrive is installed by default on Windows 10 systems, the number of icon overlay handlers that other applications can register before the problem that's described in the "Symptoms" section occurs is effectively reduced. OneDrive currently registers five icon overlay handlers, and there are four system icon overlays. This leaves just six slots in the system image list for use by other icon overlay handlers.