SLA feature was introduced in CRM 2013 but, CRM 2015 has introduced an interesting feature to pause SLA calculation for specific On Hold statuses. This essentially ensures that SLA calculation gets stopped. This can be achieved by
1. Going to to Settings -> Administration
2. System Settings -> Service Tab
3. Select valid On Hold Case Statuses to pause SLA calculation.
Note – Available Values automatically populate the status reasons added for On Hold status.