unslice has two modes of operation. If given the -f flag, unslice reads a control file telling it how to assemble the images. This is a text file with two decimal numbers on each line, one line for each slice to be assembled into the output image. Each line gives the starting and stopping scanlines (inclusive) for each slice. These must be in ascending order. This is useful if the slices have excess image area that should be cropped away.
If no control file is given, the -y flag is used. This tells unslice what the maximum Y value of the output image is. Unslice reads the files in order, using the RLE headers to determine where to place the slices. If two slices overlap, the first scanlines from the second slice are thrown away. In both cases, the slices must be in ascending order, and are expected to be of uniform width.