I can see that part (I think) in all my windows.
Is it the part that the sliding portion of your window slides into?

Yes, it is the part that the sash goes into. I guess it is just a new way of trimming, but in older windows, the stool is flush with that piece, not below it.

Is that a replacement window or original to the house?

NONE of my replacement windows go to the windowsill.

They are new windows, but not just replacement sash. They are part of a complete remodel, all windows replaced, rooms reconfigured, etc. We had a few windows replaced a couple of years ago and the were flush, so I think it has to do with the way they were ordered.

They are new windows, but not just replacement sash. They are part of a complete remodel, all windows replaced, rooms reconfigured, etc. We had a few windows replaced a couple of years ago and the were flush, so I think it has to do with the way they were ordered.

That is the way all mine have been installed--replacement and new construction--in the last 7 years.

A window "sill" is the bottom part of the window's frame and is only seen from the outside. The horizontal shelf-like trim added to the inside is called a "stool".

A traditional window stool has a notch in it that engages the back edge of the sill and rests against the window sash providing better resistance to wind driven rain.

Many modern "nail-fin" windows already have an upturn at the back edge of the sill. These windows often have a groove into which the tongue of a stool trim fits.

If you want a more traditional looking stool it is possible to use a stool without a tongue and raise it higher. This works better on some windows than others.

The lesson to learn is to never approve a window selection without seeing it in a showroom or looking at the cross-section detail of the sill/stool assembly.

This is a traditional window:

This is an Andersen window:

The stool installation depends largely on the design of the back edge of the window sill although it is often possible to make a special profile stool trim to fit many of the windows that have an integral raised waterstop at the back of the sill. However, it is usually necessary to tell the carpenter that you are willing to pay for that extra work.

I have linked to a video of the installation of a traditional window stool.

Here is a link that might be useful: window trim video