Arborists are responsible for maintaining and removing any trees on your property. Before they can do so an arboricultural report needs to be conducted. This is done in order to determine the way they need to go about their task. Safety and legal issues are the main reason for these reports. Removing large trees can be dangerous and an assessment of the situation needs to be conducted before proceeding with the task.
Source: https://homedecoratingsecrets.wordpress.com/2018/06/18/all-you-need-to-know-about-arboricultural-reports/