NOTE: the Office 365 app version does not support site columns.

The field can also be used in site columns, and, by extension, in content types.

• Create Product Categories and Products lists according to the instructions above.  You may want to create the lists in the top level site of the site collection to make the site columns available to all sites in the site collection.
• In the same site create a site column named Product Category connected to the Product Categories list.
• Create a new Master-detail lookup column in the same site, name it Product. Set the Parent Field to Product Category and Link Column to Category.
• You may now use the column in lists of the current site and its sub-sites. Please note that you must add both Product Category and Product field.
• You can also use the columns in content types. Here also you need to add both fields to the same content type.