setting of interword glue

Han The Thanh
Thu, 27 Jul 2000 10:39:02 -0400

Hi all,

can someone please explain to me the reason for setting of interword
stretching/shrinking as follow

   \setint{fontdimen(3)}{\scale{\int{interword}}{600}}% interword stretch       
   \setint{fontdimen(4)}{\scale{\int{interword}}{240}}% interword shrink        

Another question is that why it is given in ext file? Shouldn't it be 
rather in a mtx file?