Mirch

Or some of the things are just suggestions or advice.

Some things in Quran have been mandated by telling us that it is Halal or Haram . For example Blood is haram. Flesh of certain animals is halal.
For somethings it is said that they are hudood ullah ( boundaries of Allah) so they become mandatory or Farz.
For example laws of inheritance.
About somethings Allah says that they have been written for you . They become mandotory. For example Salat/namaz.
Then there is some advice. For example a Muslim male has the option to marry upto four women at a time but it is not mandatory or Farz/d.
So what makes something mandatory or farz/d on a Muslim ?
A clear order from Allah and his clearly defined boundaries , and some other things which Allah says have been written by Allah for us.
But there are many things in Quran which come under the guise of advice but many scholars and many sects have made them mandatory/fard/farz. Why is it so ?
I recon hijab will fall under the category of advice but not as a fard.
I would consider the verses which tell us that a Muslim male can marry a woman of faith but a Muslim woman should not be married to people of faith as advice too.
And the permission to males to marry upto four women at a time will be a suggestion , not even an advice.
Please correct me if I am wrong.