Rule 13 refers to the phonogram 'sh'. The phonogram 'sh' is used at the beginning or end of a base word such as she and dish, at the end of a syllable such as fin ish, but never at the beginning of a syllable after the first one EXCEPT for the ending ship such as friendship.
For example: fresh, marsh, sharp, finish, readership.
There are other phonograms and another rule for when to use the 'sh' sound in a second or subsequent syllable (Rule 14).
Watch the tutorial video below to see how the Spelling Tools, Spelling Rule 13 and Reading Tools are used to spell challenging words.
Special Note:
Not all words will follow the spelling rule. These are called exceptions. If 90% or more words follow this rule then this is still a great tool to help become a proficient speller. Language experience will deal with the rest.