When I query with the Structure Fields.I wanna to know what is the difference between On and Exact in stereochemistry options
The exact stereo option means that all stereochemical features are evaluated for equality between the query and the target. This is best illustrated by an example.
If you draw a wiggle bond then this means either a wedge bond or a dash bond, but it is not known which. With stereochemistry turned on (the default) this bond in the query will match any of these: wiggle bond, wedge bond, dash bond. But with the exact stereochemistry option set it will only match the wiggly bond.
Maybe the simplest way to think of this option to interpret the stereochemical features "as drawn".
For more information on the stereochemistry options look here: