If earlier version of Mync is being installed, this option will appear. In case of ticking the option, EDIUS 11 installer removes earlier version of Mync. On the other, if you untick the option, both earlier version and 11 version of Mync are usable.
Below are things to consider which is better selection for you:

1. Want to use Smart catalog and Storyboard
Untick the option to keep earlier version of the Mync. Because both are no longer available. Run earlier version of the Mync to use them.

2. It’s enough to migrate older library and catalog
Tick the option. Once installation completed, migrade the older database – details here

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• Way to migrate older Mync library to Mync 11