How does these work Alerts, Task Due Dates, Standard Alerts Days, Critical Alert Days in Performance

Dear Friends,

Can any one guide how the Alerts, Task Due Dates, Standard Alert Days and Critical Alert Days in Performance Works.

I have set some due dates, Standard Alert Days and Critical Days, Enabled notifications and run the process "Send performance Document Alert Notification"

But still we are not receiving any alerts via bell or email.

Please anyone explain the steps to follow to achieve task and how reminders will come or work actually?