This field is visible from within your customer portal, found in the Upcoming section. It's important to note this is only visible to your customers if you've enabled show product attributes from the Product And Shipping Management section in your Customer Portal Settings.

If this setting is enabled, your customers will see a View more button (highlighted below).

Once they click the View more button they'll see the following screen below, where they can add their own attributes to their order. This field corresponds to the Attribute Value (highlighted below).

These attributes are visible in the Order Attributes section of any given subscription order.