That's probably because certain file extensions are associated with a particular point cloud type. There are two main point cloud types, each with their own indexed file associations. For example, geographic LiDAR point cloud data is commonly associated with .xyz file format types. If you try to use a .pcd file extension, it likely will not open in certain software. I'd recommend asking what kind of scanner was used to create the point cloud. Once you have that information, you can more easily decipher what type of extension will work with the data.

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