What's the difference between 注册 and 登记, both meaning "to register"?
注册 (register) can also mean "licensed" as in  "注册医生" (licensed doctor),  "注册执业" (licensed to practice) 

We don't say 登记医生 (registered doctor) or to 登记执业 (register to practice) 

When you 注册 on a system, you are officially registered/ enrolled or licensed

When you 登记 on a system, you are either officially registered/ enrolled or submitted an application