They’re very happy. The sales have been huge, just massive. By our calculations based on player IDs, you’re looking at 300,000 in sales, which is a very significant chunk of total ArmA 2’s sales. So they’re obviously very happy about that and it’s a validation for their strategy and focus with modding.

Yes, I think modding is really good because you go along someone else’s footsteps and you can learn a lot about how someone else has done something. It’s kind of like reverse engineering things. You figure out what they’ve done, how their data structure works, how their engine works and all these other things.

I think it is a really good place to start because you’re using someone else’s framework. If you want to cut your teeth straight in there with C++ I think that’s a lot to chew off and you can end up not getting exposure to all those issues that if you knew them would make a lot more sense when building your engine from scratch or using someone’s toolkit engine from scratch.

In the video game world, a great example of this is when companies open their games up to mods, so users can tweak them or build entirely new games on top of the same basic engine. Valve's Counter-Strike series grew from a fan-made mod for Half-Life, which was so popular it has been credited with keeping Half-Life on gamers' radars for years longer than it would have been otherwise, leading Valve to hire the creators and turn it into its own game, which remains one of the company's most successful titles. This week another example bubbled up on Reddit, in the form of a captioned screenshot of the Steam store titled "Dear developers, this is why you should make your games moddable":

The game ARMA II: Combined Operations was on track to be another mostly-forgotten game, still enjoyed by a small group of fans with few other prospects. Then, two years after its release, and without getting any kind of promotional sale price, it started selling like crazy and surged to the front page of the Steam leaderboards. Why? Another team of developers One of the company's developers released the alpha of a project he'd been working on independently: Day Z, a zombie-survival game built as an ARMA II mod. Fans have been clamoring for a particular type of zombie game for a while now (and Cracked's Robert Brockway pitched a similar idea recently) and the description of Day Z sounds like it fits the bill—so when the free alpha of the mod was released, lots of people bought a copy of ARMA II so they could give it a try. The developer was expecting it to be a hit within the existing fan community, but he had no idea that it would cross over into the mainstream.

In this situation, everybody wins. Gamers get a new game, ARMA II gets renewed sales, Day Z gets to exist (without the need to build a brand new engine). The sales boost to the original might be temporary, or it might spark new interest in the game and revive it entirely, or it might inspire newer and even more popular mods, or... well, there are a lot of possibilities, none of them bad. All because the ARMA II creators had the foresight to let people add value to what they created.

Update: A commenter pointed out that Day Z is the independent project of one of the developers working on ARMA 3. Post has been updated to reflect that fact.