With this product-specific custom field you define whether the corresponding link opens in a new browser tab.
With the "inherit from theme configuration" option, the global setting of the "Further links" extension from the theme configuration applies.
Each of the three links (Further link 1–3) has its own field.
You can find the "Further link 1 – Open link in a new tab" custom field in the respective product:
Tab "Specifications" ⇒ Block "Custom fields" ⇒ Tab (Set) "Further links (... Pro)"
Type: Selection
Technical name: twt_{theme}_{edition}_custom_field__product__further_link_1_new_tab