If you've not worked with an offset printer before, you probably won't know what an overrun/underrun is. You'll commonly see this term come up in estimates/quotes when we're brokering offset projects. It'll be listed as something like:
+/- 5% Billable Overrun/Underrun
An overrun is any additional items that are printed past what quantity you've ordered. An underrun is the number of items printed short of what you'ves ordered. Using the 5% from above, if you've ordered 500 magazines you may receive, and be billed for, 525 magazines. This would be a 5% billable overrun. In the event of a 5% billable underrun you'd receive, and be billed for, 475 magazines.
This scenario happens because offset printing is not as simple as taking an item off a shelf, putting it in a box and mailing it to a customer. There are often multiple machines used to create your order and there may be some waste involved with each machine. If you ordered 500 magazines, the offset may attempt to run the project with 535 "units" in mind, knowing there'll be some loss along the way. You may end up with 525 or 475 magazines depending on how many magazines are lost along the way. The more machines involved and the higher the quantity ordered means it's harder for the offset company to end with the exact number ordered. If they end up underrunning the project by 25 magazines, it's very cost and time prohibitive to restart the job to produce the missing 25 units.
This is why the printing industry uses a + or – percentage factor when it comes to quotes. If they end up printing 525 magazines, what are they going to do with the 25 extras? They're not going to trash them. They've produced them and they need to get paid for them. Similarly, if they're short 25 magazines then that's the number that gets shipped. It's very rare to encounter an underrun, most offset printers plan well and know what kind of loss to expect.
Plan accordingly. If you need exactly 500 magazines, you may want to order 530 in case of an underrun. If your budget is tight and you wouldn't be able to afford an overrun, you may want to order 475 to be safe.
The offset partners Printigree works with usually quote with a +/- 5% billable overrun/underrun and because we're a small, independently owned business, we have to pass that along to our customers.