Apple's explanation of a planned Wi-Fi upgrade fee has its roots in obscure accounting rules that tell companies how to book sales of future product upgrades.

Apple said Thursday that it plans to charge customers $1.99 for a software download that enables the 802.11n Wi-Fi technology currently present in almost all MacBooks and MacBook Pros with Intel's Core 2 Duo processor. The company says accounting rules known as generally accepted accounting principles (GAAP) force it to ask for money for the download.

"During the past several months Apple has shipped some Macs with the hardware to support 802.11n, but the draft of the 802.11n specification was not complete enough to create the required software," Apple spokeswoman Lynn Fox said in an e-mail to CNET News.com. "Now that the draft specification is complete, we are ready to distribute the software to make the 802.11n hardware in these Macs come to life."

But because the company has already recognized all the revenue from the sales of those computers, it has to now charge customers at least a nominal fee in order to establish the value of its software upgrade and satisfy an obscure accounting regulation known as SOP 97-2, said Fox.

Apple didn't have to do it this way, say accounting experts. But the company most likely faced difficult choices in relation to the upgrade: It could have held off on shipping the new Macs until the upgrade software was ready. It could have skipped the 802.11n capabilities altogether. Or it could have deferred revenue from the new Macs until the software was ready--all unlikely and unpalatable options. Hence, the $1.99 fee.

Apple began selling MacBook Pro notebooks with Intel's Core 2 Duo processor in September, later adding that chip to MacBooks and iMacs. However, the company also included 802.11n chips in almost all of those systems without telling buyers, Apple CEO Steve Jobs revealed at Macworld last week.

Here comes the tricky part: under accounting regulations developed over the last several years, when companies sell a product with multiple pieces that are delivered at different times, they must determine the separate value of each piece of that product, accounting experts say. And the company can only record the revenue associated with a specific piece when it is delivered to the customer.

This is a very common practice in many industries, said Ryan LaFond, assistant professor for accounting at the Massachusetts Institute of Technology's Sloan School of Business. For example, magazines receive all the money for a full year's subscription at the time the subscription is purchased, but they only record that as revenue as each issue is delivered, deferring the remaining balance into a liability account called unearned revenue.

That means a company in this situation would have to defer all the revenue associated with the product until it can establish the value of the Wi-Fi upgrade, or until it delivers the complete set of software, said Brett Trueman, a professor of accounting with the Anderson Business School at the University of California at Los Angeles. So, Apple would have had to defer all the revenue for Macs sold with the 802.11n chips from September until it delivers the upgrade in February, and that's not a realistic option.

So now, Apple has to establish a value for the Wi-Fi upgrade in order to satisfy the requirement to separately account for the different pieces of software. One easy way to do that is to charge people for it.

There's absolutely nothing in the GAAP requirements that says Apple must charge its customers for that software upgrade. The only requirement imposed by GAAP is that Apple must account for the separate value of the 802.11n capability, said MIT's LaFond. It can do this by creating a value at the time of purchase or it can wait until it delivers that capability to record all the revenue associated with the product.

Another option, if the company had wanted to keep the 802.11n capabilities secret, is to create a "new arrangement" with the customer. Apple sold the customer a notebook in September, and is now selling the customer 802.11n capabilities for that notebook. These are two separate transactions that satisfy the need to account for the undelivered 802.11n capability as well as Apple's desire to book all the revenue for the notebook up front and keep the use of the 802.11n chip a secret.

Any of those options would satisfy Apple's need to account for the separate delivery times for the Macs and the 802.11n capabilities, according to several experts interviewed for this article. But simply blaming the fee on GAAP, or on the Sarbanes-Oxley regulations as some rumors have suggested, does not tell the full story.

"If I'm a company, and I want to give my customer something, GAAP isn't going to prevent you from doing that," LaFond said. But at a time when Apple's accounting practices are under significant scrutiny from regulators looking into the company's stock-options backdating practices, the company has to be extra careful about following the proper procedures while keeping financial analysts happy with strong earnings reports.