How is the field TitleID calculated?

It's the volume serial number as known from cdplayer.ini as an integer

Programmers can get the TitleID using the call

BOOL GetVolumeInformation(
  LPCTSTR  lpRootPathName,              // address of root directory of the file system
  LPTSTR   lpVolumeNameBuffer,          // address of name of the volume
  DWORD    nVolumeNameSize,             // length of lpVolumeNameBuffer
  LPDWORD  lpVolumeSerialNumber,        // address of volume serial number
  LPDWORD  lpMaximumComponentLength,    // address of system's maximum filename length
  LPDWORD  lpFileSystemFlags,           // address of file system flags
  LPTSTR   lpFileSystemNameBuffer,      // address of name of file system
  DWORD    nFileSystemNameSize          // length of lpFileSystemNameBuffer
 );