What is an active engaged contact? Open/click though/form submit?

I am attempting to find the number of engaged contacts over the last 12 months. I have run the Database health dashboard and the figure it gives me for active engaged contacts is 1/3 the number of contacts returned when i run a segment with the following criteria:

1) opened an email in last 12 months. or

2) Clicked through from an email in the last 12 months. or

3) submitted a form in the last 12 months

When i run a segment without the "opened an email" criteria it comes back with half the amount the database health dashboard say are engaged. Therefore I wonder what exactly does the database health dashboard consider as an engaged active prospect?

Also is it best practice to consider an email open as an engagement?

Thanks

Kevin