Standard Alert Days vs. Critical Alert Days

Summary

Standard Alert Days vs. Critical Alert Days

Content

In Fusion Performance, on the Performance Template, on the Process tab, you can define Standard Alert Days and Critical Alert Days.  We've tested using the Standard Alert days - which sends reminder notifications the number of days defined  prior to the due date of a task.  But I'm not clear on what the Critical Alert Days are and how they are used.  Is it just a second notification option (where the number of days defined would be less than the Standard days)?  

Or is it something completely different?

Thanks!