New to sigil magic

What makes a vowel or consonant unique? On this page that I've found concerning sigils it says to use the unique letters of your stated goal. However I cannot figure out what makes a letter unique.

http://kitschchaos.com/articles/chaosmagic_sigils.htm

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The unique letters would be letters that show up multiple times in your stated intent or goal.

The method asks that you cross out any repeated letters keeping only "unique" letters, that is to say letters that only occur once.