What it covers
The track is the channel a sliding window or door runs in. The sill is the ledge at the bottom of the frame. Neither gets any help from rain, so dust that lands there tends to stay.
Why it finishes the job
Sparkling glass above a track full of dirt still looks unfinished, and the dirt is right at the spot where you put your hand to open the window.
Sliding doors count
Sliding glass doors have tracks too. Mention yours when you ask for a quote.
Common questions
Should tracks be cleaned along with the glass?
When you open a clean window, you see the track. Ask Robert about including them in your quote.