What do "add to cart" and "in cart" mean?

What do “add to cart” and “in cart” mean?

Much like a shopping cart on commercial websites, your data cart contains the variables and samples that you want to include in your customized extract. You can place variables into (and remove them from) your data cart while browsing the documentation system by clicking the icons in the “Add to Cart” column. When you are viewing your data cart, the icons appear in the “In Cart” column and work the same way.