MAP2FS is a jiffy which converts a CCP4 map or mask to XtalView fsfour format, so it can be read directly into xfit.
MAPIN can be a CCP4 map or mask. If a mask is converted, then it will be smoothed very slightly to improve the contour mesh generated by xfit. To override this behaviour, first turn the mask to a map using MAPMASK, then use MAP2FS.
MAPIN must cover exactly one unit cell - if neccessary, extend the map/mask first using MAPMASK (XYZLIM CELL option).