Impôt proportionnelle - I don’t understand

[Headers]

What is this tax? I can’t make head or tail of any of the texts I’ve read to understand how it is being applied.
Can anyone explain it to me please?

[RobertArthur]

In France, a proportional tax is a tax where the rate remains fixed, regardless of the size of the taxable base (income, wealth or the value of an asset). Unlike income tax, which is progressive (the rate increases with income), a proportional tax applies the same percentage to everyone. Example of everyday life: the VAT rate (usually 20%) is the same whether you buy a product for €1, €10 or €1,000.

[exile]

If this post is in respect of the tax return, you can (or at least could - I have made no effort to remain up to date) use this for special one off gains. For example the one off payment taken with a commuted UK work or private pension can work out more advantageous to take as the flat rate, than to add it to your other incomes and pay on the increasing sliding scale applied there. You do need to do the sums though.

One thing I did find thanks to the very helpful fonc at the tax office: If your income from other sources does not reach the level for income tax being applied, you can put some of that one off payment onto your other incomes up to the tax free limit and then pay the flat rate only on the remainder.

[Headers]

Thankyou for responding.
I don’t have any special one off sums. I have bank interest from uk banks. Have I put it in the wrong box perhaps?