Mark Twain posted:
Right, so then in formal written English it would be improper to write the contraction, as the possessive form of the sentence is correct as formally written? Meaning, you absolutely would not write the contracted form of the word, in formal written English.
There is no absolute restriction here. It is perfectly grammatical to use [apostrophe + s] for contracting "is" or "has," and it need not always signify possession in formal writing.
However, it does tend only to be used for possession in formal writing, and where there are exceptions it is generally with "it's" and "that's". Note that "it's" is NEVER possessive when written with the apostrophe.