If you want to display popup notification you have to add groups.Now,only that user is notify which are in configured groups. As per display in screenshot this "Tasks to Complete" reminder is only visible to the users which are in configured groups.

If From Today Field is checked then the notifications that will be visible in the reminder panel will be from today.

User can provide a User domain by using the User Domain field, if the domain is given then that domain will be applied in the records that will be visible in the reminder panel.

In the Display Fields Field, you will be able to select the fields that you need to be displayed in the reminder. You can choose from the fields that would be in the model that you would have selected earlier in the Model field.

As you can see in the above screenshot when you choose days in the search option field, a new field named days will become visible. By giving relative amount of days you will be able to see the notifications for that particular time period.

As you can see the bell icon here in the main menu bar in the right screenshot, by clicking on that bell a reminder panel will pop up which can be seen in the below screenshot. You will also be able to see a counter of the notification that you may have at an instance.

As Shown in the above screenshots, We can select the Reminder from the Reminder selector. Whichever reminder is selected that particular reminder will be displayed.

Pop up will be reloaded on each action on the ODOO client. Also We can open the relevant record by clicking on the record from the reminder widget.