Whenever we correlate actions to belong together. I.e., when one action triggers another. Then we have a starting/triggering point (=source action) and an endpoint (=target action).
For example, when you click on a confirmation button on a checkout page then you often have a XHR action that triggered it (=source action), and usually either of the following:
Such action chains/correlations can be best seen in the waterfall chart!
Hence, you can use the of "sourceURL" and "pageURL", or the XHR equivalent "XHRUrl" and "Top XHR URL" to properly name your actions in cases where such correlations happened.