floating and Nonfloating I/O modules

Hello everyone,

     I'm checking out the manual fot the S5 ET 100U due to some power supply problems in the factory I work. I've worked in projects with S7 PLCs and ET 200s, but these old PLCs and modules are still operative  in the factory. I'm attaching a picture with an excerpt of the manual. What do they mean by floating and nonfloating modules, exactly?

Thank you veyr much in advance.