The iPhone example is always good for explaining this one.
An iPhone costs £1,000 in the UK (mugs for paying that, but that's another story). The components for an iphone probably cost £50. So does that mean the UK tax is on £950?
China says cool but it's Chinese labour that went into making the phone so, as well as the cost of the components we bought, add the Chinese labour and it actually cost £55 to make the phone. And we provided a "value add" service here. There are plenty of other places where you could go and get that phone made and most of those companies will make a 15% profit, so yeah, we think the Chinese share of the revenues is £65 and we'll get the tax on £10.
Then Ireland come along and say we import the phones into the EU and provide all the ongoing support for the iPhone which is part and parcel of the product. If that wasn't priced into the phone, the customer would have to pay £150 to an external party for the support, so we think we should get the tax on the £150. Oh, and our import and brokerage service, warehousing costs etc is worth another £50. So we'll have tax on £200.
And then America comes along and says, hey, the iphone is just a bundle of standard components bundled into a generic design. What people are really buying is the Apple brand, and that's American. And moreover, the brand is clearly the most valuable part of the phone. So we think the US share of profits is £500.
So the UK is left with £235. But hang on, of the £1,000 phone, £165 of that was VAT, so the profits are only £70. But also in the UK are the costs of selling the phone - shipping, store costs, advertising etc. So we're now down to £30.