Difference between grid tied and grid assisted inverter

Thanks for the question.   What they are referring to is that a grid-assisted, as opposed to grid-tied, means that excess electricity from the solar array feeds a battery bank and is not sent back to the grid.  It is also know as a grid-tied with battery back-up system. 

Here is a link to a good discussion regarding a battery back-up / grid assisted option.

"What are the options for a grid-tied array to continue to generate power in the event of a power outage?"