Do not use what in place of which. For example:
Correct: Call Type Objects are used in Security Policy Rules to identify which type of call traffic applies to the Rule.
Incorrect: Call Type Objects are used in Security Policy Rules to identify what type of call traffic applies to the Rule.
If you can drop the clause and not lose the point of the sentence, use which. If you cannot, use that. For example:
Correct: SMDR data can be used to resolve outbound calls that were ambiguous because source was unavailable during the call.
Incorrect: SMDR data can be used to resolve outbound calls which were ambiguous because source was unavailable during the call.
(If you drop the which clause, the sentence meaning changes: SMDR data can be used to resolve outbound calls.)
Correct: My house, which is the red one on the corner, is 30 years old.
Incorrect:
My house that is the red one on the corner is 30 years old.
(Implies that I have more than one house on that street, e.g., the red
one on the corner and the blue one next to it.)