How to know when to Use a Global Optionset
If you want to reuse the option set again, you should do it this way
How to know when to Use a Local Optionset
When you want it to be used for ONLY one place and never reuse it again.
When you want to publish a managed solution - using global optionsets in managed solutions can sometimes cause some problems.
How to identify a local option set versus a global option set.
With the system administrator role, navigate to Settings > Customization > Customize the system.
In the default solution, expand the components section and then click on Option Sets.
In that location you will be able to see all of the option sets which are going to be global option sets. Anything not on the list are local option sets.