Learn Italian: I parametri, i quali cambiano la procedura, sono molto complicati. - The parameters, which change the procedure, are very complicated.

You could absolutely use che here, as it is the standard relative pronoun for subjects. However, il quale and its plural/feminine forms (like i quali) are formal, stylistic alternatives. They are often used in written or elevated Italian to sound more elegant, or to avoid ambiguity because they clearly show the gender and number of the noun they replace.

How do we know to use i quali instead of il quale or le quali?

When using this stylistic type of relative pronoun, it must agree in gender and number with the noun it refers back to (its antecedent). Since i parametri is masculine and plural, we use the masculine plural form i quali.

Could we use cui instead of i quali in this sentence?

No. cui is only used when the relative pronoun is the object of a preposition (like di cui meaning "of which" or a cui meaning "to which"). In this sentence, the relative pronoun is the subject doing the action (changing the procedure), so you must use either che or a subject form like i quali.

What is the purpose of the commas around i quali cambiano la procedura?