What is the concept of valid and invalid alphabets ?

While defining an alphabet of letters consisting of more than one symbols, no letter should be started with any other the letter of the same alphabet i.e. one letter should not be the prefix of another. However, a letter may be ended in the letter of same alphabet i.e. one letter may be the suffix of another.
Σ= { a , b } ( Valid Alphabet)
Σ= { a , b , cd } ( Valid Alphabet)
Σ= { a , b , ac } ( Invalid Alphabet)