What is the meaning of <rayCount>, <rangeCount> and <laserCount> for laser sensor XML?

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Hi, I am using simulating a laser sensor. However, I have some doubts about the meaning of some tags in the xml description. Even the ROS web page do not provide information on this.

The xml description look something like this

<gazebo reference="base_scan_link">
            <sensor:ray name="laser">
                <rayCount>683</rayCount>
                <rangeCount>683</rangeCount>
                <laserCount>1</laserCount>

                <origin>0.0 0.0 0.0</origin>
                <displayRays>false</displayRays>

                <minAngle>-135</minAngle>
                <maxAngle> 135</maxAngle>

                <minRange>0.06</minRange>
                <maxRange>4.0</maxRange>
                <resRange>0.001</resRange>
                <updateRate>10.0</updateRate>

                <controller:gazebo_ros_laser 
name="gazebo_ros_base_scan_link_controller" plugin="libgazebo_ros_laser.so">
                    <gaussianNoise>0.005</gaussianNoise>
                    <alwaysOn>true</alwaysOn>
                    <updateRate>10.0</updateRate>
                    <topicName>base_scan/scan</topicName>
                    <frameName>base_scan_link</frameName>
                    <interface:laser name="gazebo_ros_base_scan_link_iface"/>
                </controller:gazebo_ros_laser>
            </sensor:ray>
        </gazebo>

I wish to know what are the function of <raycount>, <rangecount>,<lasercount> and Gaussian noise. Can anyone explain what are the meanings of the above mentioned tags? Thank you.