Clicking a link icon displays the Link Menu for that parameter:
The Link Menu shows the names of compatible GDS tags within the software. For example, the Min Blob Size parameter is an integer, so it can be linked to integers and longs.
The Link buttons for tool outputs appear in the right column of a tool within the tool list:
Hovering the mouse over the link buttons identifies their associated parameter as shown below. Clicking the link button displays the Link Menu populated with GDS tags of a compatible type.
Once a parameter is linked, the background of the link button turns green.