What happens when you declare more than $10,000 at customs?

Could you please elaborate on the implications of declaring more than $10,000 in cryptocurrency or other financial assets at customs? Are there specific penalties or regulations that one should be aware of? Additionally, how does this declaration process differ from traditional currency or other types of financial instruments? Is there a standard reporting threshold across various countries, or does it vary? What documentation or proof of ownership is typically required for such declarations?