Please take a look here: Additional Variables in the Data Set
The variable LASTPAGE tells you which page was answered (sent via "Next") most recently before the browser tab was closed. If someone decides that they do not want to do the questionnaire on page 2, then LASTPAGE has the value 1, and consequently all data from later pages is missing.
If someone clicks "Next" on page 2, but has not answered any items there, all the items have the code -9, and LASTPAGE is 2. The page is shown again, if respondents must answer the questions, but ... if they close the window instead of answering, this is the data.