The Capital API is so complicated. What do I put in the config for selector?

Just leave it empty. Every user may have a different selector, e.g. users using the Currency schema may have {currency: xxx}|, users using the World schema may have an optional {world: xxx}`, etc.

But I don't understand what's a schema

You don't need to understand. Just leave the config entry there so that users who understand can edit it.

But my plugin doesn't support multiple currencies/worlds/accounts

You automatically support multiple currencies/worlds/accounts as long as you allow the user to edit the selector config.

But how can the user make multi currency when I don't understand it?

Maybe... because the user is smarter than the developer? Things... do happen.