Apple announced a series of big changes at the executive level of the world's most valuable company on Thursday, on the same day that a report indicates sales of the company's biggest moneymaker, the iPhone, could be slumping. Here's a rundown on how the pecking order in Cupertino is changing, and what it means.

On Thursday, Apple CEO Tim Cook announced he finally had an official second-in-command at Apple.

According to a statement from the company Thursday morning, former VP of Operations Jeff Williams has been appointed as Apple's Chief Operating Officer. Meanwhile, Phil Schiller, senior VP of the company's global marketing, will take over an additional role as head of the App Store, while relative newcomer Johny Srouji has been promoted as Senior Vice President of Hardware Technologies.

Jeff Williams as COO

"We are fortunate to have incredible depth and breadth of talent across Apple's executive team. As we come to the end of the year, we're recognizing the contributions already being made by two key executives," said Cook in the company's announcement. "Jeff is hands-down the best operations executive I've ever worked with."

Williams has been at Apple since 1998, and since 2010, has been in charge of overseeing the company's expansive global supply chain. His appointment can be understood simultaneously as unsurprising but also significant.

First of all, the significance of Williams' appointment becomes clear when you consider that Apple's current CEO held the same position when Steve Jobs was running the show. Indeed, experienced Apple watchers, like 9to5Mac's Mark Gurman and The Register's Simon Sharwood, see the appointment as an exercise in "succession planning." The thinking is basically, if Williams continues to master Apple's critically important global supply chain (as Cook did under Jobs), the next and only step beyond that is CEO.

On the other hand, Williams -- often referred to in the hallways of Cupertino as Tim Cook's Tim Cook -- is mostly just being officially named to the position that he's worked in since Cook left his COO position and took over for Jobs in 2011.

As Cook's role as the face of Apple has expanded over recent years, Williams has gradually taken over more responsibilities overseeing the global operations of the company. The promotion likely gives Williams a big bump in pay and keeps him tied to the company for years to come, but it also just makes what Williams has been doing for years an official position. 

Schiller Takes the App Store From Eddy Cue

The second major change in Apple's top leadership lineup is the addition of overseeing the App Store to Schiller's many duties as head of Apple Worldwide Marketing.

"Phil is taking on new responsibilities for advancing our ecosystem, led by the App Store," wrote Cook in the announcement, "which has grown from a single, groundbreaking iOS store into four powerful platforms and an increasingly important part of our business."

Schiller will be charged with extending Apple's software ecosystem, as he now heads almost all of Apple's developer relations across its hardware platforms.

Unstated by Apple in Schiller's promotion seems to be a demotion for senior VP of Internet Software and Services Eddy Cue. He previously headed the iTunes Store, the App Store, the iBooks Store, iCloud, Maps, Siri, Apple Pay and nearly every other software platform run by Apple.

As Gurman pointed out, Cue's official bio on Apple quietly changed after the announcement, taking the App Store and iBooks Store out of his long list of responsibilities. Forbes speculated the change from Cue to Schiller may be due to increasing disappointment and criticism from developers who think Apple's mobile software platform has declined in some respects recently.

Johny Srouji Turns Senior

Johny Srouji has spent a mostly quiet eight years as Apple's VP of Hardware Technologies, and was just promoted to the senior level in a nod to his importance to the company's future.

Srouji led the development of Apple's "A" series of mobile systems on a chip since 2010, when the "A4" was introduced as the company's first self-designed system on a chip.

Since then, according to Apple's statement, Srouji "has built one of the world's strongest and most innovative teams of silicon and technology engineers, overseeing breakthrough custom silicon and hardware technologies including batteries, application processors, storage controllers, sensors silicon, display silicon and other chipsets across Apple's entire product line." 

With the senior level promotion, likely including a huge pay raise, stock options, and executive perks, Apple is seeking to keep Srouji with the company well into the future -- a future that's driven, after all, by hardware innovations that set Apple apart other technology companies and are largely responsible for the company's bottom line.

Fresh Leadership, Refreshed Apple?

Apple would never publicly acknowledge that changes needed to be made, but the shakeup of its executive leadership comes at a time when analysts and investors are beginning to feel a decline at the company.

On the same day that Apple announced the executive changes, the Wall Street Journal reported that the company's stocks hit a two-month low during the week, likely because of an increasing cautious outlook by analysts on the future of iPhone sales.

The consensus on Wall Street is that Apple will only gain about 3 percent in sales from the previous year, as opposed to the 37 percent increase seen last year, after Apple introduced the first large-screen iPhone 6.

Some analysts -- six out of 32 polled by FactSet, to be exact -- even predict that this fiscal year will be the first in which iPhone sales fall, year to year. Morgan Stanley is one of them, which predicts a six percent decline in sales of iPhones, Apple's top product that drives almost 60 percent of the company's revenue.

The pace of innovation, or lack there of, also has Wall Street setting lower expectations for Apple going into 2016. Investment bank Raymond James, for example, released guidance on Wednesday stating, "There have been many notable supply-chain suppliers to Apple in the last few weeks to either pre-release, report or guide to weaker-than-expected results," which "suggests modestly fewer iPhone upgrades are likely in the coming three and 12 months than a year ago."

With Apple's stock nearly flat on the year, the new executives will certainly feel the pressure to prove Cook right for putting them in charge.