Normally, any places on the puzzle grid that are unused are filled with characters taken from the letters used in the puzzle, in the proportion to how often those letters are used.

This provides several features:

However, for certain simple puzzles, such as one made up of words such as: hat, cat, bat, fat, etc., the traditional way of filling in characters doesn't work as well.

Selecting the Fill with English letters option fills all missing places in the puzzle with random letters from 'A' to 'Z'.