Signature Description

enum class io_format : unsigned char  {
    csv = 1,    // DataFrame specific csv format
    csv2 = 2,   // Regular csv format (similar to Pandas)
    json = 3,
    hdf5 = 4,   // Not Implemented
    binary = 5, // Not Implemented
}; 
This specifies the I/O format for reading and writing to/from files, streams, etc.
Currently only CSV, CSV2, and JSON formats are supported. The CSV format is as follows:
Any empty line or any line started with # will be ignored

See read/write docs for format specifications
C++ DataFrame