Source means original media
Select Source to inspect a library item and mark its useful range. You are choosing what to bring into the edit, not trimming an existing timeline instance.
Select Sequence to navigate and review the assembled film. Check the viewer label before using In, Out, or placement commands.
Read the range boundaries
In is the first included position. For source movies, Out is an exclusive boundary: the selected range stops just before it. Sequence Out displays the last included frame. Read the displayed range label rather than assuming these are identical conventions.
Source timing can retain finer precision than sequence frames. Numeric controls are useful when a boundary must be exact.
Find the original again
Select a timeline clip and use Match Frame (F) to find the corresponding source moment, or Reveal Source in Browser (⇧F) to locate the library item.
If a command appears unavailable, check selection, viewer context, and whether a text field still has focus. A selected source and a selected timeline clip are different inputs.