ximtoppm creates a PGM (portable graymap) file containing the alpha channel values in the input image. If the input image doesn't contain an alpha channel, the alpha-filename file contains all zero (transparent) alpha values. If you don't specify --alphaout, ximtoppm does not generate an alpha file, and if the input image has an alpha channel, ximtoppm simply discards it.
If you specify - as the filename, ximtoppm writes the alpha output to Standard Output and discards the image.
Actually, an Xim image can contain an arbitrary fourth channel -- it need not be an Alpha channel. ximtoppm extracts any fourth channel it finds as described above; it doesn't matter if it is an alpha channel or not.
See pnmcomp(1) for one way to use the alpha output file.