For those of you not familiar with the term, filing the serial numbers off refers to guns and crime. Generally to make it harder to trace a gun to it's owner a criminal will file the serial number off the weapon. It's an older practice and I'm not sure how often it's done anymore, but the term is one that will sometimes crop up in gaming when it refers to someone using an idea or parts of an idea from one source in their own source.
For example, I have a Pathfinder campaign I run where I pull elements from several other D20 game settings. As I've run it, I never bother to rename the race or item I lift from another source to drop into my Pathfinder setting.
Recently though I thought I would try my hand at writing the setting up and publishing it through one of the online game material sites. It was only as I went over my notes and logs that I realized just how much terminology and substance from other sources there was in my setting.
Obviously I don't want to be someone who steals the hard work of other writers for their own enrichment. I do plan to continue to use the core ideas, but I will obviously have to change the names and enough details to make them my own. File the serial numbers off as it were.
But what if you're just running a game for friends. Can you use other things, like the name of a race and it's particulars? Chances are if you're just using it for your group you should be fine. I'm not a lawyer, so this article should not be considered legal advice in any way.
What if you want to use something, but you want to try and hide it from the players. Maybe they all know that the second you describe the outfit the NPC wears they will know who it is and what role they play in the story? In cases like this, I suggest boiling it down. Not your players, the idea.
What about the idea interests you? Find the core of the thing. Use that and then let your own imagination fill in the gaps and details. Make it as much your own thing as you can. They say that there are no really original ideas. It's all, they say, in the presentation.
Guns for example. My Pathfinder campaign setting starts with cannons that use black powder. It's dirtier than real black powder and only used in cannons because there is no safe way to use it in smaller weapons, and it's a dirty fouling substance. Cannons have to be cleaned after every shot, they don't have great range. They are mainly used against large troop formations or fortifications.
Part way through the campaign the PCs encounter shipments of something coming from somewhere in Africa (I use a modified Europe as my campaign setting). This strange powder burns it it comes into contact with Abyssinian Gold (brass that is 90% copper and 10% zinc).
Some time later they come to learn that the Prussians have invented a weapon that uses paper packets of this powder with a lead slug at one end, essentially a needle rifle. So we bring rifles into the game.
The original powder was in fact two powders that were from the Iron Kingdoms setting. Over time I changed the idea to provide me with the same results, but filing the serial numbers off the other games element.
Most of the time I will happily drop something I've lifted in homage from another source right into my game. Many times the players recognize the thing and usually enjoy it. Although I have to admit I may have used Star Gates a little too much over the years.....Maybe.