IT projects and methods seem to big on the word "customer" these days...
Common definitions of:
customer - one that purchases a commodity or service
purchases - the acquisition of something for payment
So let me propose this definition: unless you are selling software as a product for real money, your IT project does not have any customers.
I have spent my career working on IT projects for companies that need systems to run their business more effectively/profitably.I have worked with many a sponsor, and many business people who will use the systems, but none of them paid me directly for doing so; we all worked for the same organization, with job titles and salaries to go with those titles.
Most of these organizations used some kind of budgeting/cost allocation to align the cost of the systems to the revenues of the parts of the company that would use the systems. However, this is not purchasing, it is management.
However, some of these organizations used a definition of an internal form of exchange - 'gray money' or better known as "funny money" - to not only allocate cost but to measure managers' effectiveness and often their bonuses. The main company I saw this at is no longer an independent organization, as it fell on hard times and was acquired/assimilated by a more successful company. One of the reasons was management choices like the following:
Two similar business units (same product, different market) needed new systems. One (Unit A) got a head start and developed a functional system. The other (Unit B) saw the system and said "we could use that too". Unit A asked for payment in funny money of half the cost of development. Unit B balked, and spent a lesser amount of real money to buy a package, because that looked better on Unit B's cost reporting... real money spent that should not have been necessary.
So, within an organization, there are no customers and there are no purchases; acting like there is can be detrimental to the organization as a whole.
But we are talking about customers of IT projects, surely that can't be the same; yes it is. Call someone a customer and they will act in their own best interest, however that is measured, and with disregard for the success of the whole organization.
IT projects should not have buyers and sellers; they should have teams of business and IT people working together to reach a common goal. So you see, the definition of "customer" in an IT project is that there isn't one.