AFAIK tonemapper is like an S-curve that maps HDR to LDR, no? So what’s a tone curve? And does it apply before or after tonemapper?
I was also looking into this and found this page of the documentation: https://dev.epicgames.com/community/learning/knowledge-base/BLEJ/unreal-engine-color-pipeline-basics
If you scroll down further, you can see a graphic that splits the “Post Processing” step into a smaller “Post Processing” and “Tonemapper” step. The Tonemapper than includes the “Filmic Tone Curve”, which is probably generally referred to as just “tone curve”.
I am no expert on the topic, but to answer your question: I think the tone curve applies during the tonemapper.