Hi,
MinTurningRadius is a property, Default value is 4. You can refer the following documentation linkhttps://www.mathworks.com/help/driving/ref/pathplannerrrt.html
In general the formula for radius of curvature is as
r = s / (sqrt(2 - 2 * cos(2*a/n))
where r is radius of curvature ,s is wheel base, a is steering angle,
n is steering ratio.