setting of interword glue

[Han The Thanh]

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?

Regards,
Thanh