It is very unclear. The BIM Server INCLUDES the BIM Server Manager.
Install the BIM Server if you want a server on your machine where you will host files.
The BIM Server Manager is installed when you will only be logging into someone else's server. For example, if the file you will be working on is on your colleague's machine or the office server, you only need the BIM Manager on your computer.
If at a later date you decide to host files on your computer, you can replace the BIM Manager with the BIM Server.
Clear as mud?
MacBook Pro Retina, 15-inch Yosemite 2.8 GHz Intel Core i7 16 GB 1600 MHz DDR3
Mac OSX 10.11.1
"Implementing Successful Building Information Modeling"