All customers will be selected by default. If you want to choose customers of a specific segment or segments: 

  1. Click the Edit option of All customers
  2. Select In specific segment(s) 
  3. Click on the Please Select box to see the available segments
  4. Select your preferred segments

You can either include customers of selected segments or exclude. All filters that you apply after that will be only on the selected customer base.

If you want to switch to all customers, click the respective edit icon and select All customers