The inception date of a smallcase is the date from which the smallcase's index value is tracked.
You can view the inception date by clicking on the tooltip (question mark) above the 'CAGR'
This opens up the 'Help' pop-up where you can view the inception date of the smallcase. In this case, this smallcase is being tracked from June 29, 2007.
This index value calculation includes the rebalances from the date of inception, hence this helps you understand how the underlying model has performed in the different market phases.