[Pharo-users] What is "Fluid"?

I have attached 2 cropped screen-captures of the same class displayed in the class browser; class-smalltalk.png is the regular class syntax, class-fluid.png is the same class but with the "Fluid" checkbox selected, and shows a different syntax. May I know what that is all about? There's a different terminology too, something called "slot", which I have faint memories of being quite like that in "Self" environments. Thank you, ~Kathe