Where would sourcing a TMC fall in the category mapping?
It very much depends on how much you spend, how critical this supplier was to your organisations success and how many alternate suppliers were able to deliver what it is you needed. Let me give some examples....
If you were an oil company spending $200M on travel / $5M on TMC, travel was a critical part of your business getting employees on and off oil rigs and your systems were integrated with the TMC in order to create a highly efficient process for your travel bookers, then you may fee that this should be a 'Strategic Differentiator' category and require a lot of attention and cultural alignment would be required.
Conversely, if you were spending £200k on travel with pretty straight forward transactions and little integration required or needed with the TMC then this would likely be assumed to be a leverage supplier.