The map data_type describes the data structure of the map voxels.
(CCP4 map header word 4 MODE)
EMDB currently holds MODE=0,1,and 2 maps; the majority are MODE=2.
MAPS with MODES other than 2 and 0 may not work in CCP4 programs.
MODE = 0: 8 bits, density stored as a signed byte (-128 to 127, ISO/IEC 10967)
MODE = 1: 16 bits, density stored as a signed integer (-32768 to 32767, ISO/IEC 10967)
MODE = 2: 32 bits, density stored as a floating point number (IEEE 754)