In-app purchase within an iOS app, an inclusion that could cost some developers an extra licensing fee

Apple is said to be "actively investigating" the patent claims made by holdings firm Lodsys, which last week began sending a handful of developers notices that their apps were infringing on a patent the group held, The Guardian reports.

Just yesterday Lodsys posted a series of frequently asked questions, along with corresponding answers to its blog, wherein it detailed how its patent pertains to the in-app purchase feature of iOS, and the developers who use it. The group seeks an ongoing, revenue-based licensing fee, as well as back payment on apps that make use of the feature.

So far Lodsys has not made mention of how many developers it's sent notices to. Those who received letters from the group last week noted that Lodsys was giving them just 21 days to license their use of in-app purchase with the threat of legal action if they didn't.

In yesterday's FAQ the group said it was effectively going after all apps that make use of IAP, no matter if the company was big or small. "From a fairness perspective, we have decided that Lodsys should attempt to license all users of the patent rights, on proportional terms, rather than let many 'free riders' not pay while only selected companies pay," the group wrote.

Lodsys' claims could have a considerable impact on the appeal of developing on the iOS platform. While app makers are expected by Apple to secure the rights to trademarks, copyrights, patented technology, and any other intellectual property within their applications, Lodsys is going after developers for something Apple has built into its own mobile operating system. This is also of special importance for Apple to keep around because the company gets 30 percent of the revenue from each IAP.

Lodsys wrote in its missive that Apple's current license for the technology doesn't allow it to pass that on to app makers who then sell on its platform. "The value of the customer relationship is between the Application vendor of record and the paying customer," the group wrote. "The OS (is acting as an enabler) and the retailers (are acting as a conduit to connect that value), and taking their percentage for that middleman role."

With that definition, others like Google and Microsoft, which both operate mobile software stores could have their developers targeted by Lodsys if those apps feature in-app purchasing tools.

Apple and Lodsys have not responded to multiple requests for comment. Lodsys said yesterday that it's not speaking with press about the matter.