Coordinate system of pointcloud

When using ROS camera, we get XYZ information of each point. What is their coordinate system? I learned from the Internet that the positive direction of X in most point clouds is in front of the camera. Y is to the left. Z is up. But when I browse the point cloud data, I find that the X information in the point cloud has a negative value. Why?

The point cloud follows the ROS frame standard coordinates system described in the REP103 document, so X forward, Y left and Z up.
You can get negative X values only if you are getting point cloud coordinates referred to other frames than the camera frame.