WM_ICON_SIZE
The ICCCM property on the root window where the window manager says which icon sizes it would like — one of the rare cases of information flowing from manager to program.
Most ICCCM properties are set by the client and read by the window manager. WM_ICON_SIZE runs the other way. The manager puts it on the root window with XSetIconSizes, as a list of minimum and maximum width and height plus increments in each direction, and a program about to create an icon pixmap for WM_HINTS reads it with XGetIconSizes and draws accordingly. If the property is missing, the program guesses.
On 1980s and 1990s screens it mattered: twm and mwm showed icons in fixed slots, and an icon of the wrong size got cropped or floated in a sea of background. Few programs bothered to ask. With EWMH's _NET_WM_ICON the client ships ARGB images in several sizes and lets the manager scale, which made the property effectively redundant.