Hi Conan,
I am not super well versed in how the ranking works with service types, but I did find some information in the documentation wiki regarding it:
Manage Service Type and Service Type Events - CA Service Management - 14.1 - CA Technologies Documentation
From what it looks like, the lowest ranked service type would be used. Now I am not totally sure if its the lowest ranked for each object, meaning the lowest ranked one from the category and the lowest ranked one from the priority (meaning both are enforced), or if its only the lowest ranked one over all.
Additionally, regarding options manager - there is an option called cr_allow_sla_downgrade which appears to have an effect on the service type behavior based on rank. Here is the info I gathered from the "help about this option" screen:
cr_allow_sla_downgrade
Alters the behavior of the CR SLA Option by allowing the system to downgrade a request Service Type automatically.
The CR SLA Option selects the best Service Type from among several request attributes, but never replace the request current Service Type with a type of lesser rank. If this option is installed, the Service Types for all affected attributes are evaluated whenever one of the attributes changes. The request Service Type is set to the highest ranked type found, even if the new Service Type is lesser in rank than the request current Service Type.
The Service Type with the smallest Rank value is considered the best service. If all the Service Types considered are equal in Rank (including Service Types with empty Rank values), the Service Type created first in the database is selected.
The CR SLA Option must be installed for this option to function correctly.
I hope this helps you out a bit more.
Thanks,
Jon I.