In most cases it is recommended to create a field for your custom data and assign it a key. Creating a field with a key ensures that the key for your data will remain consistent.
However, if you wanted to add a note to an order but wanted to do so in a more structured way, or add different notes to different products, then you may want to set a custom key.
When creating your field, choose the Custom key option. This will allow you to enter in a custom key and value when entering in your custom data.