Apple (NASDAQ:AAPL) earnings are coming up -- and lots of investors will be watching. Scheduled for Nov. 1, the earnings release will give investors a better idea of how sustainable the company's reinvigorated growth rates are. Both Apple's revenue and earnings-per-share growth rates have accelerated recently.

Adding to the pressure is the impressive performance Apple's stock has seen. Shares are up 38% over the past 12 months. Investors will be looking for a stellar quarter.

While investors will want to check on the usual items, such as revenue growth and iPhone sales, when Apple reports earnings for its fourth quarter of fiscal 2018 on Nov. 1, there's one key metric that may steal the show: revenue guidance.

Here are several reasons investors should watch Apple's revenue guidance closely.

A man holding Apple products in an Apple store

Image source: Apple.

Guidance will preview Apple's most important quarter

First, Apple's fiscal first quarter, which is the period for which the tech giant will provide guidance in its fiscal fourth-quarter report, is seasonally the company's biggest quarter. This, of course, is due to the timing of Apple's product launches and the holiday season. To highlight how big the quarter is, 35% of Apple's trailing-12-month revenue came from the company's fiscal first quarter of 2018. 

Since the period is so important, guidance for strong year-over-year growth in fiscal Q1 typically translates to a good year for Apple.

Checking on iPhone

Second, Apple's guidance for its fiscal first quarter will give investors an idea of how the company's newest iPhones are selling. Since the iPhone accounts for over 60% of Apple's revenue, management's forecast for overall revenue is highly dependent on what Apple is expecting from iPhone.

The trajectory of Apple's revenue guidance relative to the year-ago quarter, therefore, gives investors a window into the general trajectory management expects from iPhone revenue.

Can Apple keep up its strong growth?

Finally, Apple's guidance will be watched particularly closely this time around because the tech giant's strong return to growth has given investors high hopes for the iPhone-maker's growth potential over the long haul.

Since returning to growth in the company's first quarter of fiscal 2017, Apple's year-over-year revenue growth has accelerated for an impressive seven quarters straight. Even more, Apple has now reported four quarters in a row of double-digit revenue growth -- and the 17% year-over-year revenue growth the company posted in its most recent quarter was its strongest revenue growth in 11 quarters.

With such strong momentum in its revenue growth, investors will be watching guidance to see whether Apple looks poised to keep up its strong growth, but there're doubts about the tech giant's ability to do so in Q1.

If analysts are right, investors shouldn't have their hopes up for guidance for another quarter of double-digit revenue growth. On average, analysts expect Apple to guide for fiscal first-quarter revenue of $92.5 billion, implying 5% year-over-year revenue growth.  

Daniel Sparks owns shares of Apple. The Motley Fool owns shares of and recommends Apple. The Motley Fool has the following options: long January 2020 $150 calls on Apple and short January 2020 $155 calls on Apple. The Motley Fool has a disclosure policy.