Lets consider a scenario. We would like to use an option set attribute say Validity Range on many entity forms. In simple words, we need to define this option set globally and then attach it the attribute on the form. Global option set could be found in when you expand the Solution as shown below.
Choose New option to define a new Global optionset.
Suppose we would like to reuse this option set on the contact form. So lets traverse to Contact entity and choose fields.
Choose 'New' to add a new attribute.
Choose the Type as Option set. Here comes the twist. You could see an option 'Use existing option set'. This determines whether we are going to use the global option set or a new one. Also please note that the form layout change when you choose 'Use existing option set' to Yes.
Another key advantage of this feature is that we are able to define 'Requirement Level' in our new attribute as per our choice. At the same time we could reuse the values from the Global Option Set.