For the purpose of this jam:
If you're creating an SRD for the jam, we mean it in the very technical sense of the term. Make a System Reference Document rather than just a TTRPG that you deem hackable.
If you're making a TTRPG for the jam, some creators have given permission to hack their games and have not made an SRD. These are usually one-page RPGs and the mechanics are basic enough that they don't feel the SRD is necessary to make it easier to hack. You're free to hack these or use mechanics from them in combination with something that does have an SRD. If someone doesn't have an SRD and you're not sure if they would mind if you hacked their game or not, please reach out to them first and also clarify how they'd like to be attributed if they say yes.
Beth will be updating the rules of the jam to make that clearer. Thanks for reaching out for clarification.
~Angel