Half of the clues have an extra letter coming from the wordplay, and the other half have an extra letter in the clue itself. The former ‘spell out an interpretation to be placed on the grid. The latter ‘spell out what must be highlighted in the completed grid, and why five of the 17 cells involved must be distinguished from the other 12.’ The completed grid will also have ‘a thematic comment that identifies the distinguishing feature’.
How very curious.