How to use H3LIS331DL 3-Axis Linear Accelerometer with arduino to detect Harsh braking of vehicle
A harsh braking is nothing more than a strong negative acceleration, so you could compare your accelaration values to a threshold. Let's assume, that the positive x-axis is the moving direction of your vehicle. This means, that braking will give an acceleration in the negative x direction. Then you first define a specific threshold for the acceleration. An acceleration in the negative x direction above this level means, that it is a "harsh" braking (you will have to test yourself, what value you want for this). Then you test, if the read values are below the negative of our threshold (since acceleration in the negative direction will be presented by negative number).

int braking_threshold=1000;
...
void loop(){
    ...
    // Convert the data
    int xAccl = ((data[1] * 256) + data[0]);
    int yAccl = ((data[3] * 256) + data[2]);
    int zAccl = ((data[5] * 256) + data[4]);

    if(xAccl &lt; -1*braking_threshold){
        // Do whatever you want to do at a harsh braking here
    }
    ...
}


Note, that often the accelerometer data is rather noisy, especially, when mounted on a vehicle, that will vibrate during the motion (due to motors and such). You might need to apply some filter to the read values, before you can use them. That can be as easy as averaging some values, but can also go on to more complex filter algorithms, depending on how noise free you need the data. You will have to test, what values you realistically get in your application and how noisy they are. But data noise reduction is a complete different topic and thus off-topic for this question.

Also note, that your accelerometer measures a range of 200g. That's quite much I would say. When measuring relatively low accelerations the signal to noise ratio will be rather bad. So I hope, that you really want to measure accelerations, that fill this range, rather than only using a very small percentage of the range.