Each custom link has the following attributes that you can configure on this form :

Link title
  The text of the link. This can include substitutions like $DOM and $USER, which are replaced by details of the selected domain.
Destination URL
  The full URL that the link points to. Again, substitutions like $DOM can be used.
Open in window
  If Same is selected, the link will open in the right-side frame. If New is chosen, it will open in a separate top-level browser window.
Category
  If you have defined any custom categories for links, you can select a category for each link in this column.

The domains and users that links appear to can also be controlled using the following fields on this page :

Virtualmin user types
  To limit links to the master admin, domain owners or Virtualmin resellers, check the appropriate boxes in this field.
Virtual servers from template
  Only when a virtual server created using the selected template is chosen from the left menu will this link be visible.
Virtual servers with feature
  Only when a virtual server with the selected feature is chosen will the link appear.