The Trimmer window has been a part of VEGAS Pro since the very first version that featured a video editing tool set. However, the window has lead a somewhat conflicted life.
Most users might never even use the window. After all, the very innovation that made VEGAS Pro an instant and enduring success was the efficiency with which the user can edit directly on the timeline. No longer was it necessary to bring your video into an old-school source window first to define in and out points. Instead, VEGAS Pro made it possible—and in many cases even preferable—to drag a file directly to the timeline, quickly trim the clip event down so that it includes just the portion of the clip you want, and then get on with the rest of your edit. One never even need touch a source window.
Read More:
https://magazine.vegascreativesoftware.com/2017/01/05/trimmer-window/