Prescriptive things have to do with giving rules or directions. Prescriptive advice tells you what to do.
When the doctor writes a prescription, he or she is telling you to get a particular drug and take it. Similarly, anything prescriptive involves telling people what to do. People are prescriptive when they're sure of what’s right. "You have several options" is not prescriptive, but "You should do this" is very prescriptive. A bossy person is very prescriptive.