joint.dia.Link is the basic model for diagram links. It's a Backbone model with couple of additional important properties. The first one is again a unique identifier for the link stored in the id property. The rest can be put into three groups:

Connections

{
    id: <id of an element>,
    selector: <CSS selector>,
    port: <id of a port>
}

{
    x: <number>,
    y: <number>
}

link.set('source', { id: element.id })
link.set('source', { id: element.id, selector: 'text' })
link.set('target', { x: 200, y: 300 })