What does "P" mean? "P" is for "panel mount" in this product. This Grove – Light Sensor (P) is a sister version of original Grove – Light Sensor. They are identical except the Grove connecter is moved to the back so that you can easily use it as a neat and wire-free human interface device. For all Grove users (especially beginners), we provide you guidance PDF documents. Please download and read through Preface - Getting Started and Introduction to Grove before your using of the product.