The product whose page should display the button is the product that you'll attach the tag to. For example, if you have a laptop product which has the tag "spiff-personalisable-product-handle-laptop-bag", then when customers visit the page for your laptop product they can launch a workflow for your product with the handle "laptop-bag".
Both the customized product and the product for the page will be added to the cart, then the customer will be redirected to the cart page.
The customized product will be added to the cart but the product for the page will not be. The customer will then be redirected to the cart page.
The customized product will be added to the cart but the product for the page will not be. The customer will not be redirected away from the page.