Hi,
If you activate that option, the module will display its elements based on the current main element displayed on the page.
So on the product page, it will display the elements based on the product of that page.
On a products listing, it will display the elements based on the current category of the listing.
And if the option is deactivated, the module will instead use the "main category" of your module as base for loading its elements.
So yes, for a related products module, you want that option activated.
Regarding the module settings you can read this tutorial:
www.hikashop.com/support/documentation/3...splay-front-end.html