Activity custom fields and task and event page layouts
Key Takeaway
You create custom fields for tasks and events on the shared Activity object, so each field is available to both. You decide where a field appears by adding it to the task page layout, the event page layout, or both.
Tasks and events are both activities, and they share one underlying object called Activity. You create custom fields for both on Activity, so a new field is available to both, and you use the separate task and event page layouts to decide where each field appears. Some standard fields belong only to tasks or only to events, which affects both layouts and formulas.
Tasks and events share the Activity object
Because tasks and events share the Activity object, you create their custom fields on Activity. A new activity custom field is available for both tasks and events unless you use page layouts and record types to control where it shows.
Tasks and events each have their own page layouts, so you use these separate layouts to decide where each field appears.
If you want an activity custom field to show only on tasks...
Add the field only to the task page layout.
If you want an activity custom field to show only on events...
Add the field only to the event page layout.