ISS Fan Club

On the NASA site there is an aid to finding the ISS as it goes over in the night sky. It has both tabular data and a sky picture called SkyWatch which shows a visual of the track of the ISS across the sky. (BTW SkyWatch is gotten by clicking on SkyTrack.) Of course this is old news to folks here.

I was going to explain it to my sister, but I am puzzled by one aspect of SkyWatch. When I am out looking at the sky, searching for the ISS I am remembering that the track drawn on the SkyWatch visual has two (sometimes 3) parts. One part is called acquisition and one is called sighting.

What in tarnation is the difference? When, where, what should my eyes be looking for? The acquisition track is in red and the sighting track is in yellow, but I'm really quite certain the ISS doesn't change colors from red to yellow to red as it flies over.

Of course one could ask why NASA doesn't include an explanation of the confusing picture, hopefully without reference to the Acquisition Patent blather (and the very minimal LEGEND about it) they display, but that's another story.

Help?