Apple's (NASDAQ:AAPL) iPhone product category has fundamentally reshaped the company over the past 11 years. Thanks in large part to this product franchise, Apple is the most valuable publicly-traded company in the world, commanding a market capitalization of roughly $1 trillion.

The iPhone business achieved record results in Apple's fiscal year 2015. iPhone unit shipments surged 37% to 231.2 million in that year and iPhone revenue grew by a staggering 52%, hitting $155 billion -- thanks to both this robust unit growth and a higher average selling price (helped by the introduction of the company's larger and pricier iPhone 6 Plus).

Apple's Jony Ive and Tim Cook, looking at red and yellow iPhone XR smartphones

Image source: Apple.

iPhone unit shipments and sales both dropped sharply in the company's following fiscal year, to 211.9 million and $136.7 billion respectively, before inching up to 216.8 million and $141.3 billion respectively in fiscal year 2017.

In fiscal year 2018, Apple's iPhone business had its best year ever -- at least in terms of the most important metric. Let's take a closer look.

Record revenue, not record units

In its fiscal year 2018, Apple shipped 217.7 million iPhones, just a smidgen higher than what it shipped in fiscal year 2017 and almost 3% more than what it shipped in fiscal year 2016. You'll note, though, that this is still below the unit shipments record that the company set during its fiscal year 2015.

While Apple wasn't able to top its fiscal-year 2015 iPhone shipments in its fiscal year 2018, it did manage to set an all-time record for iPhone revenue at $166.7 billion -- up about 7.5% from the fiscal-year 2015 peak. A significant increase in the company's iPhone average selling price allowed Apple to do this without shipping a record number of units.

As you might recall, Apple began selling the (now discontinued) iPhone X, a device that cost $999 for the 64GB version and $1149 for the variant with 256GB of storage capacity, on Nov. 3, 2017. This was during the first quarter of Apple's fiscal year 2018. The success of that device -- which, in the words of CEO Tim Cook, "changed the industry, and along the way became the No. 1 smartphone in the world" -- was instrumental in pushing up iPhone average selling prices over the course of fiscal year 2018.

Still hugely dependent on the iPhone

During Apple's fiscal year 2018, the company generated $265.6 billion in revenue and $59.5 billion in net income -- both all-time highs for Apple, and both helped substantially by the success of its iPhone business.

Of that $265.6 billion, $166.7 billion -- roughly 62.8% of the total -- came from iPhone sales. For some perspective, that's an increase from the approximately 61.6% of revenue that the iPhone made up during the company's fiscal year 2017, although it's below the 63.4% it provided in fiscal year 2016 and the eye-popping 66.3% of sales it accounted for in fiscal year 2015.

Apple is still extremely dependent on the performance of its iPhone business. This is true even as it enjoys growth in both its services business (up 27% year over year last quarter, excluding "a one-time favorable adjustment of $640 million recognized in the fourth quarter of fiscal 2017") and its wearables business (which Cook said had "revenue growth over 50%" last quarter).

Apple began selling the iPhone XS and iPhone XS Max on Sept. 21, and the less-expensive iPhone XR on Oct. 26. I look forward to seeing how those devices perform over the course of fiscal year 2019, and what the company has in store for its all-important iPhone product lineup in the years ahead.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.