Professional expenses

Dear Sir/Madam,

As a self-employed person, I believe you have the choice of proving your expenses or using a flat rate, which is 30%. Is that correct? Could you explain how this works? After all, you have to prove your expenses for your vAT declaration, don’t you? Or is that 30% for expenses such as electricity, transport, etc. that you paid for as a private individual but that you also use partly for your self-employed in secondary occupation? Thank you for explaining this to me.