Looking for documentation on % strings in X programs

[D. R. Evans]

When setting up dynamic wallpaper, the various commands include
placeholders such as %x and %f.

For example, one command to run xplanet on the desktop is:

 xplanet --geometry %xx%y --num_times 1 --output %f.jpg && mv %f.jpg %f

Does anyone know where the precise meanings of all the possible
placeholders is documented?

(For example, it seems that %x must mean "get the current width of the
screen".)

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