John Matonis defines ecash as portable if:
The security and use of the digital cash is not
dependent on any physical location. The cash
can be transferred through computer networks and
off the computer network into other storage
devices.
In their test stages, none of the ecash I experimented with
was transferrable to any other denomination or off my computer.
The ecash system Matonis would probably recommend would
incorporte ecash with cash cards and cash card readers so that
a user could transfer cash from card to computer and vice versa.