Any Board Member (or higher permission level) can add custom fields to a board and also set whether the values of those fields show on the task and/or on the task face.
When you add Custom fields to a Board, the field will be set to "Show" by default when tasks are added to the board. Think of this as a setting that will set up the custom fields to your preference on ALL tasks added to the board.
If, for example, you remove the custom field from the board, it will still exist on all the board's tasks, but won't be set to show on new tasks going forward.
Adding a Custom Field to a Board
To add a custom field to a board, open the board and go to the Settings
Then navigate to the Custom Fields section in the Board Settings navigation
Click in the search field to find available custom fields
Note: Custom fields must be created by a Custom Fields Manager before being available to add to a board
Click on the custom field you want to add to the board
Displaying Values on the Task Face
To display values of a custom field on the task face, click on the setting cog icon and select Show on Task Face
The custom field value will display on the task face like this
Global Custom Fields
If a global custom field is set, by default it will show in the custom fields section on every board. Global custom fields cannot be removed from boards and will display in all tasks and subtasks system wide.
If a global custom field is not set to show on all task faces, you can set that at the board level.