SYS:Devs/DataTypes contains the descriptors (these are little IFF files which describe the file format and tell the system which DataTypes decoder to use.
The decoder libraries themselves are stored in SYS:Classes/DataTypes
For a single DataType decoder library, you may need any number of descriptors. Some base class ones (eg. binary.datatype) don't need any, others may have more than one (if they can decode a few formats) - however usually it is one descriptor per decoder (.datatype file)
