Metadata

Represents a metadatum of attribute

Metadata is set per attribute, not per product, so if you use the same attribute across multiple products same metadata will be sent with every product that uses this attribute.

Metadata is a feature that allows for attribute description with Key > Value list, so it can be used in different cases, and it's really up to the user how he will do it.

The analogy to this would be a product that is described by a list of attributes: attribute code > attribute value.

A usage example would be the possibility to reflect Magento 2 one-to-many attributes to groups relation.

magento_groups -> group1,group2

or

magento_group1 -> group1

magento_group2 -> group2

But it is NOT specifically connected to any Magento functions like attribute groups.

Basically, those are data that allow for some description of the attributes.

Technically it's simply the next attribute data type