Jorge, I copy the online description for this option here "
Enables the "classic" model of service types and SLA's. This model uses the Rank value for Service Types to determine the correct Type for a ticket. Only one Service Type can be applied to a ticket.
Note: The Chg SLA Option, CR SLA Option, and Iss SLA Option are available if the Classic SLA Processing Option is installed.
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so you can see with this option installed, only one service type is allowed and it is determined by the rank(lower rank overwrites the higher rank)
Thanks _Chi