key property

Attribute name.

For example: color, brands, attributes.custom_attribute, such as attributes.xyz. To be indexable, the attribute name can contain only alpha-numeric characters and underscores. For example, an attribute named attributes.abc_xyz can be indexed, but an attribute named attributes.abc-xyz cannot be indexed. If the attribute key starts with attributes., then the attribute is a custom attribute. Attributes such as brands, patterns, and title are built-in and called system attributes.

Required.