[Jprogramming] Fill Value

[June Kim]

  (<0), (2 1$1 ; 2), (3;4)
+-+-+
|0|0|
+-+-+
|1| |
+-+-+
|2| |
+-+-+
|3|4|
+-+-+

Why are some fill values empty box while the other fill value is a box
with zero? I thought the fill value for boxes is an empty box.