When a website says a stock is "halal", it is applying a rulebook. For many screens, Sanad included, that rulebook is Shariah Standard No. 21 from AAOIFI, the Accounting and Auditing Organization for Islamic Financial Institutions, which is based in Bahrain. Its title is "Financial Paper (Shares and Bonds)", and it sets out when it is permissible to own and trade shares.
What the standard says
Owning a share means owning part of a business, so the first question is the business itself. A company whose main activity is prohibited, such as conventional banking, alcohol, tobacco or gambling, is not permissible to hold. The second question is how the company is financed and what it earns. The standard accepts that many ordinary companies carry some debt and earn some interest, and it limits how much: interest-bearing debt must stay under 30% of the company's market value, and income from interest and other impermissible sources must stay under 5% of total income.
What happens with the small impure amount
A company can pass and still earn a small amount of impermissible income. The investor's share of that amount should be given away. This is called purification, and it is part of holding the stock rather than a reason to avoid it.
What the standard does not decide
Scholars differ on details. Some index providers use different limits or measure the ratios against different denominators, for example total assets instead of market value. Those are legitimate differences of opinion, and a stock can pass one screen and fail another. An automated screen also cannot know everything about a business, so it is a starting point rather than a fatwa.
Why we use it
AAOIFI is widely recognised by Islamic banks, regulators and scholars in the Gulf and beyond, and its thresholds are published. That makes results repeatable and easy to check. You can see the working for any stock on its Sanad page.