This is a replacement window sash stop for windows with 1" channels. This part goes at the very top of the channel, to prevent the window from opening too far and potentially coming off of the balances. These parts regularly get damaged from people opening their windows too forcefully.

These parts are not structural, so they can be printed in just about any material you choose. They could be exposed to UV light if your windows do not have integrated UV filtering. If unsure, I would recommend PETG or ASA for the best longevity.