This article contains an example of a custom field that you can create for tasks.
You can add as many custom fields to a type, as required.
For more information, see Create custom fields.
Following is an example of a task custom field, named Priority. You can use it, if you need to assign priorities to your tasks:
UI Label: Priority
Description: Set the priority of this task
UI Selector: One option list
Option 1: High
Option 2: Medium
Option 3: Low
Select target type: Task