Here's my perspective on Demand and Supply in IT Service Chains, starting with:
1. Information Demand - Somebody who needs information to support their (business) activities and who is inherently responsible for specifying their needs.
2. IT Services Demand - Somebody who is tasked from a demand perspective with getting these needs fulfilled, either by manual means or by procuring IT services, to which purpose they specify the required IT services and ensure that appropriate IT services are procured.
Although 2 provides value to 1 (you could say he/she performs a service) I'm not inclined to call this a demand supply relationship. This does however apply to the relationship between 2 and 3:
3. Custom IT Services Provision - Somebody who takes on the responsibility of providing IT services that fulfil the specific demand requirements; this usually entails utilizing basic IT components (hardware, software, data and facilities) and/or integrating IT services that are either provided to fulfil specific requirements (provided by other Custom IT Services Providers) or are standard IT services that are also offered to other parties.
The 'custom' bit is essential. I'm delivering to your specs, not mine. I often call these the Intelligent Integrators. That's not the case with the next role, which takes another perspective:
4. Standard IT Services Provision - Somebody who is responsible for providing IT services according to specifications that the provider determines. It's up to the procurer to decide whether it's an good fit with what they want. I often call these parties Anonymous Artists because they make clever stuff that fulfils a market demand but they aren't particularly engaged with individual customers (think about apps providers via an apps store).
If 3 engages 4 to provide services, is this a demand supply relationship? I suppose it is but in order not to confuse with the 'principal' demand supply relationship I'd call it something like sub-supply. Don't know whether this term will catch on but you get the drift.
Now we've dealt with IT services there are two roles that supply IT components:
5. Custom IT Component Supply - Somebody who is responsible for providing IT components (usually applications) including support and according to demand specifications
6. Standard IT Component Supply - Somebody is responsible for providing IT components (HW, OS etc) including support and according to specifications that the supplier determines.
We could continue the chain all the way back to mining the raw materials but let's stop here.
Finally we revert back to the starting point with an obvious role and and an often underestimated role.
7. IT Service Use - Somebody who actually uses and therefore realizes the value that the first two roles defined.
8. IT Services Utilization - Somebody who ensures that the IT services are consumed in an effective and efficient way and the value is maximized.
So I make a distinction between
a) Demand (I want something that provides value)
b) Custom Supply (I'll give you what you need)
c) Standard Supply (I provide stuff that may (or not) fulfil your need)
d) Consumption (I realize the intended value)
It's only taken me a third of a century in IT to come up with this so it may still be immature. I'll revisit it when I reach my half century.