"That" and "which" both introduce a clause describing a noun, but they do different jobs — and mixing them up (or getting the comma wrong) is one of the most common small errors in professional writing.
"That" introduces a restrictive clause — information the sentence needs in order to identify what you're talking about. Remove it, and the meaning changes or becomes unclear. No comma before it.
"Which" introduces a nonrestrictive clause — extra information, a side note. Remove it, and the sentence still clearly means the same core thing. Always preceded by a comma.
If you can delete the clause and the sentence still identifies the same specific thing, use "which" with a comma. If deleting the clause makes the sentence lose essential information about which thing you mean, use "that" with no comma.
Getting it wrong can genuinely change the meaning. "Employees that missed the deadline will be notified" implies only some employees missed it. "Employees, which missed the deadline, will be notified" (wrong usage aside) reads as if all employees missed it. The restrictive/nonrestrictive distinction is carrying real information, not just following an arbitrary style rule.
British English is more permissive about using "which" for restrictive clauses without strict enforcement of this distinction. The rule above reflects standard American English convention, which most style guides (AP, Chicago) treat as the default for formal and professional writing.
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