You're writing your paragraph wrong - the last sentence was incomplete. Something like "No apostrophe is needed when using the possessive its." would have made

You're writing your paragraph wrong - the last sentence was incomplete. Something like "No apostrophe is needed when using the possessive its." would have made the object of your sentence much clearer. One of those other basic but trial-and-error rules of English: don't implicitly refer to something that's a long way away (in this case 4 phrases distant) from your sentence or someone will misunderstand you.