What is the difference between sensor "update_interval" and "filter throttle"?

I have a power metering sensor(this is just an example, it can be other sensors) and I’d like it to send updates only if:

  • last value was sent 20s ago
  • or if value has changed by more than 20% and last value was sent 1s ago
  - platform: hlw8012
    model: BL0937
    update_interval: 0.1s
      name: "Power"
          - median: 
              window_size: 7
              send_every: 4
              send_first_at: 3
          - throttle: 1s
          - or: 
            - throttle: 20s
            - delta: 20%

After testing, this code works as expected. But there is one thing I don’t understand, What should “update_interval” be set to?

IMHO update_interval to be less then throttle period to have at least one reading.
As well it will affect over which period median will be calculated.