This catagory of signage is purely for asethetics and the signs within it are not required to conform to any standards or regulations.
Mandatory signs are made up of white images and text on a blue background. Typical applications include 'wear safety helmet' and 'keep clear' They are there to instruct the reader to take a mandatory action.
Safety signs are made up of white imageing and text on a green background. They are either square or rectagular. Typical appliactions include first 'aid rooms' and 'fire exits'. Their meaning is to illustate a safe condition, escape route or safety equipment.