Field help (AWPDATREL) 

This field contains the date upon which the last signature reminder has been performed for the notification. It is not assigned if no signature process has been implemented.

In fact, when the Workflow event expects a signature :

When a reminder of this type is made :

If these batch rules are not used and if no other rule has been defined to change these fields, the number of reminders remains equal to 0 and the reminder date remains empty, because no other standard Workflow process can modify this value.