The importance of each policy type will depend on what features your SfB tenant is leveraging. In my experience the 4 most commonly used SfB Online policies are:

There are many policies (about 12 that applicable to individual SfB Online users), and each type of policy can have many settings (52 for the Conferencing Policy for example), and it is difficult to know what settings are available, what the individual setting enables/disables, and which ones matter the most. As I work more with enterprises adopting Skype for Business (SfB) Online in Office 365, many questions arise about setting user policies which govern which features which compliance, security, and resource usage.