If you need a little extra descriptor for the products in your build-a-box UI you can use product metafields to add your own subtitles.
1. Add a new product metafield in your shop settings
Open the "Custom data" tab (previously labeled "Metafields") in you shop settings
Now click on the "Products" row in the Metafields list.
(if you're showing individual product variants in your build-a-box UI, then you'll need to click the "Variants" row instead of or in addition to the products row)
Click the "Add definition" button in the top right of the screen
Here are the values you'll need to enter into the "Add product metafield definition" form:
Name: any name that you'll recognize later (ex "Build-a-box Subtitle")
Namespace and key:
bundle.subtitle(it's important that its this value exactly)
Description: any description that you will understand later (ex: "Subtitle that will appear under the product title in the Build a Box UI")
Type: Single line text (one value)
No click the "Save" button in the top right corner of the screen.
2. Add your subtitle to each product you want to update
For each product you want to add a subtitle to, you'll need to run through a couple simple steps:
Open the product dashboard and scroll to the bottom to the Metafields. There you'll find the new metafield that you set up in step 1. If you don't see it then you might need to click on the "Show all" button and scroll through the entire list of metafields to find it.
(if you're showing individual product variants in your build-a-box UI, then you'll need to click Edit next to the variant that you want to add the subtitle to and scroll to the bottom to find the Variant metafields)
3. Refresh your build-a-box page and see your subtitles
Wait 2 or 3 minutes and then open your build-a-box and refresh the page. Some times you have to wait longer because product data gets cached by Shopify and the Shopify might not be sending your build-a-box UI the latest product data.