Where I worked a typical call from a large customer would be:
This is Jack from XYZ, I need a hydraulic filter for unit #17
Put customer name and unit number in system. Pull up details about "as shipped" and transfer S/N to parts look up screen. Compare multitude of choices and narrow down to proper filter.
OR:
This is Jack from XYZ, I need a #1234567 and bill it to unit #17
Done, boom, 90 seconds later someone is retrieving the filter from the shelf.
Typical small customer or new customer:
I want a filter for an ABC model.
Which filter?
Hydraulic.
Do you know your serial number?
No, they are all the same, just get me a filter
Did you buy it here? I can probably look up the S/N, we really need it to get the right part.
No, they are all the same, don't you know what you are doing?
Pull up the diagram for model ABC, Go to filters and show the screen that lists 3 different hydraulic filters to the customer.
Customer pulls out cell phone and calls home. I need the s/n from the ABC because these guys don't know what they are doing.
Alternative scenario after the second "no, they are all the same" response:
Parts person sells the customer a random filter that is at best an educated guess. Then moves on to the next customer in the queue.
Customer usually returns with the "wrong filter" and either the number off of the old filter or the S/N while yelling that the people here don't know anything.
Oh, and for customers with an account, we always tried to collect as much info about a new (to us) machine and add that one to the database. Not possible for cash customers.
Sheesh, I'm glad I am retired....