Ok. look, which is mostly used in GMAT in Modifier SCs and you will not most of the time see it in other SCs.
Which mainly refers or introduces things that modify things and almost always brings trouble in gmat. in general grammar it might be different with additional explanations.
Which is removable almost always
ex.Top GMAT score, which is required by may top schools, is hard to achieve.
now remove which, and see how it works
Top GMAT score, required by may top schools, is hard to achieve.
you even do not need to use it but it mainly refers to things: colors, houses, equipment, holidays (event) and so on.