If you set a ticket to "pending auto close+" and the date is reached a cronjob (if enabled) will close the ticket with state "closed successful". If you used the "pending auto close-" the ticket will be set to "closed unsuccessful".
The first one is used for example when you send a response to your customer with a solution and your customer doesn't answer. Then you might assume that you proposed the correct solution. Hence the ticket is successful.
The second one is used for example when you request for information and you say "please send me those information within the next four days. Otherwise we won't be able to work on the issue". When you do not get the information the ticket can't be successful
