The deferred period is how long you are off work before benefit starts. Insurers offer periods from 4 weeks to 52 weeks, and the longer the wait, the lower the premium. Match it to your sick pay: if your employer pays you for 13 weeks, a 13 week deferred period avoids paying for cover you do not need.
Your job sets your occupation class. Insurers group occupations into classes, and riskier work costs more. The definition of incapacity matters too. Royal London Ireland, for example, pays where you cannot perform the essential duties of your normal occupation. We compare both across insurers.