In simple setup cases, you will be able to use one SLA for all your tickets/issues, this is the default SLA.
In some cases you might want to use multiple SLA's. For example if you have different support levels, such as GOLD, SILVER or BRONZE, for your customers.
To use multiple SLA's you need to use contracts.
A contract makes it possible to connect another SLA than the default SLA to a customer.
On the contract you select the customer and the SLA that should be used for this customer.
On the contract you can also set what products/services that the customer uses and that the SLA should be used for.
If you want to use multiple SLA's for one customers you need to have multiple contracts with different products/services set.