Task of CTL
 
  Cluster extraction
      Cluster is minimal character sequence with which we can decide their graphical shape.
  Character reordering
      Unicode basically adopts a logical order which doesn't match with the visual order of many Asian scripts.
      OpenType fonts don't have a table to perform this.
  Character to glyph mapping
  Glyph substition
      Decide which segment of sequence to apply which OTF's GSUB features in what order for ligature, etc.
  Glyph positioning
      Final step to apply OTF's GPOS feature to adjust the position of, mainly, combining glyphs.