A "file name" is a String.  Example: "C:\\MyDir\\MyFile.q" .  A "full file name" is a String that begins with a device letter and colon, e.g. C: .  A "relative file name" is a String that does not begin with a device letter and colon.  Relative file names that begin with ~ are completed to full file names by erasing ~ and prefixing the "Quantum directory".  Relative file names that do not begin with ~ are completed to full file names by prefixing the "current directory".

GetCurrentDirectory   The "current directory".
GetQuantumDirectory   The "Quantum directory".
    Usually "C:\\Program Files\\Planet Quantum\\Quantum 2011"
GetWindowsDirectory   Usually "C:\\Windows"
GetSystemDirectory   Usually "C:\\Windows\\System"

The "current directory" is whatever you like.  You will find that if you right click your mouse on the "Quantum.exe" icon on your desktop and select "Properties" from the pop up context menu, that there is a "Start in:" edit field on the "Shortcut" property sheet.  You can reset the current directory of a running session of Quantum by using either Cd or SetCurrentDirectory.

Escape sequence \\ produces a single \ character in the memory representation of a String.  This is why you need \\ in the Read representation of Quantum file names.