This has been asked and answered before. You are allowed to copy the text from the SRD, nothing else. Rules mechanics are not protected by copyright however (only rules text), so you can include mechanics similar to those in official Free League games, or any other RPG for that matter, as long as you rewrite the mechanics in your own words.I still have two questions that are not expressly and clearly outlined in the SRD, and it would make it much simpler if it was spelled out in the SRD.
1- Can I use the rules from other games, like MYZ or Alien, if I remove all IP references? I understand its not the goal of Frea League to rewrite every rule for us and include it in the SRD. But can I say use Mutation Points? That term itself is generic enough. I mean what else what I call mutations and be clear without doing backflips? What about talents? Those arent included, can I use Talents so long as I rewrite the IP specific language?
2- You mention VTT is included but it doesnt say things like an online character builder. Or a tool that lets me choose and or add specific rules that I have cleaned up of IP specific language. This woudl be really sueful but its not expressly covered or worded in a way that makes it feel safe to build.
The SRD as it stands is very stripped down and makes it feel like we have to invent a lot of new stuff vs using existing rules sans the IP, which would be preferred. I tried to explain this and ask on the Facebook page for Free League and got a round about answer that circled back to the SRD is what we can use word for word. WHich I perfectlty understand that. but can I use other rules minus IP was never answered.
You seem to be asking for a definition of what "rewrite in your own words" means. That will always, in the end, be a judgment call on a case by case basis. Can you make a game that has mutations? Sure. Can you make a game that has something called Mutation Points? No problem. Can you make a game that has mutations, Mutation Points, and a list of mutations with names and functions exactly like in Mutant: Year Zero? Maybe, but that's approaching the limit, I'd say - and not in the spirit of the license. The idea is to empower creators to make their own Year Zero Engine games, not copy official titles.
If your idea here is to create an online character builder for our official titles, then that's not within the scope of this license. The purpose of the license is to allow creation of new games using the Year Zero Engine, not tools for our existing official titles.