Sebastian,
The important thing to note is that if you decide to go with multi tenancy then things from Tenant 1 will not be able to see anything tenanted to Tenant 2. Since it's a single company it's very possible that some items might be shared.
I tend to think for your purposes data partition constraints would be easier to put into place.
I'd suggest creating separate Incident Areas with a clear title name like "HR - XYZ", "HR - ABC". Then create a View constraint on the Call_Req table for "category LIKE 'HR%'" Associate the constraint with any Role who shouldn't see HR tickets.
People will be able to create tickets using the Area, but won't be able to view them after they save and close the screen.
You'd then just need a Role set up for HR users that doesn't have this restriction.