Global fields in ACF work similar to Shop fields and the two can be used interchangeably. One crucial difference is that global fields can be referenced from field definitions belonging to other scopes such as products, collections, pages etc. (see related article below).
This allows you to use global fields as puzzle pieces for building your field definitions for a specific scope, making it a breeze to maintain cross-site content of any type.
You define and edit global fields as you would for any other scope in ACF. This allows you to store content globally available (within your shop, that is)
Custom fields for the global scope are saved as shop-level metafields and have their namespace locked to globals. You cannot create global fields with other namespaces than the above and you cannot use the globals namespace for shop-level custom fields.
Accessing global metafields
To access a global custom field value, use this construct:
Referencing global metafields
See related article below