A process can involve events as well as tasks. Events can have sub-types that you can use in a filter, or dependency, to determine whether a step has or has not started.

To add a sub-type to the Event Page layout:

To add the Process Details custom button to Process Object Page Layout(s):

  1. In Setup, use Quick Find and enter Event Page Layouts.
  2. Select Page Layouts. 
  3. Select Edit next to the Page Layout you will modify.
  4. Drag and drop the Sub Type field to the desired location on the page layout . 
  5. Select Save or Quick Save and repeat for any other page layouts that should contain the field.