For questions about various alternative set theories substantially different from ZFC. For example, NF and NFU, IST, ETCS, SP, AST.

For questions about various alternative set theories substantially different from ZFC. Examples might include:

- New Foundations (NF and NFU)
- Internal set theory
- Vopěnka's alternative set theory
- Scott–Potter set theory
- Elementary Theory of the Category of Sets (see also nLab article)