For example this could be useful if you had a list of Helpdesk support tickets. You could configure a View to show high priority, open tickets. You could then configure the reminder web part to send an email for every ticket in the list (always) every hour - ensuring that high priority support tickets do not get overlooked.

See Common Scenarios for more detail on this example and other examples of common uses for SharePoint Reminder.

When field X is ...

This option configures SharePoint Reminder to check a field in your list and only send an email if it is Due within or Overdue by a certain time.

You can choose from the following options

Due within a certain time, e.g. Due within the next 5 days.
Due between a certain time, e.g. Due between 10 and 5 days.
Overdue by a certain time, e.g. Overdue by 5 days or more.
Overdue between a certain time, e.g. Overdue between 5 and 15 days.
Due Today

See Common Scenarios for examples of common uses for SharePoint Reminder.

Note - If you choose When field X option then if the value of this field is changed it will reset the count of how many emails have been sent.

Note - If you have chosen to Expand Recurrences then the "Always" and "Overdue by X or more" options will not be available and you will be limited to the Start Time and End Time columns.