With the Post Submission Message setting is turned OFF:

• When a survey is submitted by default unsubmitted questions (non-required) are shown to the respondent. 
• The questions that have responses are submitted and showing the remaining unsubmitted questions allow the respondent to submit the remaining questions.  
• Once all the questions are submitted a default thank message is displayed.

You can change this behaviour by setting a post-submission message in the Campaign Manager.

• Click the Other setting for the campaign and toggle the 'Post submission message' button.
• You can enter the thank you message here. 
• Basic HTML can be used in the message. 
• Clicking outside the box automatically saves the setting. 

Once turned ON - if a respondent can submit a survey it will show this Post Message Message and not un-submitted (non-required) questions.