The first runnings is the volume and gravity of wort released from the mash tun before any sparging or lautering. Since no additional water is added, this measurement is useful to determine apparent grain absorption (to refine your equipment values), and conversion efficiency during the mash.
The subsequent runnings are after any sparging.
If, in your process, you don't drain your mashtun but instead add water directly above the grain bed without draining the first runnings, I would recommend at least taking a gravity sample to confirm conversion. You won't be able to determine your grain absorption accurately, but when fly sparging the volume calculations are less significant at strike/absorption level since you just keep adding water till you're arbitrarily "done".