If you have a:
1198C SI
998C SI
898C SI
798CI SI HD
798CI SI
798C SI

5.210 includes: 
The new Contour Mode has been added.  This Mode is part of the SI Enhance Menu in the Express Sonar Menu.

ContourOff 

Contour Off shows you the standard Side Imaging view

ContourModeOff

Contour On removes the water column and shows you a flat representation of the bottom

ContourModeOn

Addition of Side Imaging Range Lines

SIRangeLines

Side Imaging Navigation - When you are Navigating (GOTO) to a waypoint the boat icon on the top of the side imaging screen will try to show you the direction to the waypoint. This icon will move around to provide the direction to the waypoint. 

SINavigation

Addition of Down Imaging™ Zoom

DIZoom

If you have a: 

1198C SI
1158c
998C SI
958C
898C SI
858C
798CI SI HD
798CI SI
798C SI
788CI HD
788CI
788c
778C HD
768

5.210 includes: 

Resolution to Interlink issues affecting GPS connectivity and waypoint sharing.  Both units connected through the Interlink must have 5.210 software loaded. 

Minor Downrigger fixes were made for those with Cannonlink.

If you have a: 
998C SI
958c
898C SI
858c

5.210 includes: 

Left card reader update – allows you to see a card that has been locked on the 998c SI and 898c SI – this issue was only reported on these models.