There are few page of documentation you can consult about The Newsletter plugin. The basic plugin is free, while the Professional Extensions require a paid membership to get them all.
The Newsletter Pro plugin is the OLD professional version: all its features can be obtained with the Newsletter free version and the Newsletter Professional Extensions.
The Newsletter Professional Extensions are not single plugins but add on packages that must be installed following the instructions that can be found here.
I love the free version of Newsletter. I want to make sure I understand all/most of the sections in the Newsletter plugin, and then I know I will love having the addon, mainly for the autoresponder to setup newsletters in advance.