ACF calculates a 1 dimensional autocorrelation function.
If the dataset has more than one dimension, then the user must select along which axis the autocorrelation is to be performed. The output will contain an autocorrelation along this axis at each point specified by the other axes. Thus ACF can be run on an image series to produce an autocorrelation function at each point in the spatial plane.
Note that ACF can only autocorrelate along regularly binned axes.