This setting allows you to control what comments are pulled from Salesforce.com to JIRA:
All - All comment will sync from Salesforce.com
Private only - Only private comment will sync to JIRA issues
Public only - Only public comment will sync to JIRA issues
Regarding Comment Synchronization
Prior to version 5.3.6, all comments (whether public or private) were synchronized by default.
As a consequence, if you change the Pull comments from Remote System settings in this situation, all comments made prior to updating to version 5.3.6 will still be viewable.
Also, do take note that in situations where you have enabled the Public only option and synchronized your JIRA instance with Salesforce.com, and then changed the setting to Private only, all public comments will still be viewable.