One of the enhancements that came in SLA (CRM 2016 Update 1), which I feel has got unnoticed by few, is choosing the SLA manually using the SLA lookup field.
As we all know SLA comes already enabled for Case Entity and can be enabled for the below entities
Account, Contact, Order, Invoice, Quote, Opportunity, Lead, Activity Entities.
Basically, the above entities have SLA lookup field already created for them.
We can also Enable SLA for custom entities and activities, which will create the SLA lookup field in it.
Add the field on the form and select the appropriate SLA, and that selected SLA will be applied to that record.
It saves us from defining multiple SLA Items for an SLA to be applied as DEFAULT or to use the entitlements to achieve the same.
Hope it helps..
Nishant Rana's Weblog 
Can we have multiple SLA lookup fields on a single record and have two sla items from different sla running in parallel?
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You can only have one SLA running on one record. When an entity record is updated by using a different SLA, the previously applied SLA is canceled. https://docs.microsoft.com/en-us/dynamics365/customer-service/define-service-level-agreements#how-is-an-sla-applied
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