On Monday, Apple (NASDAQ:AAPL) will hold its annual Worldwide Developers Conference, or WWDC, kicking it off with a keynote to make some announcements. Here's what investors can expect from the event.

Apple CEO Tim Cook at an Apple event. Image source: Apple.

Software, software, software

Apple investors and customers probably shouldn't get excited about seeing any new products. This year, rumors point to an event strictly focused on software.

New versions of iOS, OS X, watchOS, and tvOS are expected to be introduced during Monday's keynote, according to MacRumors' 2016 WWDC rumor roundup. Here's a summary of some of the most-interesting rumors ahead of the event:

A Siri SDK: Perhaps one of the most-prominent rumors is that the Apple voice assistant, Siri, may get a software development kit, or SDK, which will enable the voice assistant to work with third-party apps in more-integrated ways.

MacRumors explains:

Developers will be able to build Siri integration into their apps, allowing Siri to access data and perform tasks within apps. Third-party app access has the potential to greatly expand what Siri is capable of and could put the personal assistant on par with more robust solutions like Google Now.

Siri for Mac: Siri is widely expected to finally come to OS X, which would mean Mac users would finally have access to Siri on their Mac devices.

Updates for watchOS and tvOS: Rumors about specific features that may come to watchOS or tvOS haven't gained any traction ahead of the event, but Apple does promise on its website that attendees will "learn about the future" of these operating systems -- so there certainly will be new features announced.

Why WWDC is more important than ever

Apple's iPhone sales represent well over half the company's sales, and an even greater portion of its profits. With growth coming to a halt, and even becoming negative in the company's most-recent quarter, Apple's progress in software is becoming increasingly important. Without the iPhone serving as a catalyst for revenue growth, investors are relying on Apple to continue improving software in a way that both keeps customers loyal, and better monetizes them.

Apple recently acknowledged the growing importance of software and services by introducing a new metric it calls "installed base-related purchases." This metric accounts for revenue from services like iTunes, Apple Music, Apple Pay, and apps.

Apple App Store. Image source: Apple.

Apple's installed base-related revenue increased 27% in Q2 compared to the year-ago quarter. In comparison, Apple's iPhone revenue declined 18% during the same period. Improved user interfaces in the company's operating systems, and applications running these services, is critical to driving Apple's installed base-related purchases higher.

Apple's WWDC keynote will take place on Monday morning at 10:00 AM PST at the Bill Graham Civic Auditorium in San Francisco. The event will be streamed live on Apple.com, and on Apple devices.

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