In this window you must declare the value of all the assets that you currently own , wherever they are, if you are resident in Spain and are going to pay tax by “ Obligación Personal” or just the value of the assets you currently own in Spain, if you are not resident and are going to pay tax by “Obligación Real”.
The value is calculated using the “Impuesto de Patrimonio” rules ( Cadastral value for real estate)
Any value below the lowest amount is irrelevant and you are only obliged to declare it, without the need for any other proof.
Take into account that if you already share ownership of any of the assets which are going to be acquired by inheritance, you have to take the value of your share into account when you calculate your current wealth