This is a new addition (field) to the Area 51 process, so the expectations of what should go herethere are intentionally still somewhat free form, and will vary by application and answer.
Here is what the new fields ask for currently:
...
When you submit a proposal for a site, it is generally assumed you have access to an audience to build it. Area 51 cannot find that community for you.
Please describe or link to the community where you will be publicizing this proposal:
I have checked the directory of Stack Exchange sites to see if my subject might already be on topic elsewhere.
Submit
In an ideal scenario, the proposal author should be able to respond with something like this:
This is being organized by the [American Society of Interior Designers] and is being discussed here: <https://link> (for example)
I know that is not always possible, but do the best you can. Right now this is the best indication we have to distinguish between a community that is making preparationpreparations but hasn't yet started… versus something that looks more like this: "everyone will use this bigger than stackoverflow please create" (unfortunately, we do get a lot of no-effort proposals like this).
Do the best you can.
If all goes well, I do have plans to improve the functionality based on initial feedback — including the ability to go back and edit that field to let the community know where they can go for more information and where they can rally and evangelize support. That information would become part of the proposal itself (public). No specific plans yet regarding when/if that might happen.