By default, an SLA policy gets added to each ticket the moment it gets created in your helpdesk. This is used to notify you when tickets in your helpdesk requires your attention. Setting up SLA notifications so nothing slips through the cracks.
Deciding whether to enable or disable the SLA notification for a status is very important, here is why. Let's say your team is busy attending to an emergency situation and have little time to overlook what's going on in your helpdesk, likely chance for an SLA violation is very high.
To avoid such problem, you can create a custom status and disable the SLA notification so tickets that are placed into that status will not be violated regardless of how long your agents takes to resolve the issue.