Hello!

The context of my question: I am an EU national taxed at source residing on the permit type B in Vaud, and I am married (the spouse was unemployed and we did not hit 120K in 2020). Thus to me filing a simplified tax declaration in 2020 was optional, however I did file the declaration reporting transportation and meal at work deductible costs.

I have received the simplified declaration back a few days ago and I have discovered that the calculation method for approved deductibles has been the following (I've corresponded with the tax office),
- an X amount "that was already assumed"
minus
- things I declared

I've still managed to get some little money back, and I am wondering what is this X amount, what does define it? Is it something an employer estimates and considers when paying the salary? I haven't found it in my salary certificate, for example.
I've found the official booklet for 2020 consisting of roughly 70 pages, and I've tried to skim through that (my French isn't that good) and I haven't figured it out. I'll give it another try but maybe someone knows about it as it must be something basic.
In principle, I am curious how does X come to existence, is it an amount communicated between the employer and the tax office, or it is based on a scale relatively to my income etc.? Next time it could help me to estimate whether I am better off to default instead of filing the optional declaration.

Thank you in advance!

With taxation at source, they already deduct a lot. Not 3a, however.

In short, download VaudTax and try to make a normal tax return, than you'll learn a lot.

Some "standard" deduction levels are assumed to simplify things (the X value you mentioned) which you can claim and no question is asked. When you are filling normal tax declaration you have the choice to claim a standard deduction or deduction based on your real expenses. It's a bit silly thing, they could just lower the taxes a bit and get rid of the assumed standard expenses.