I think I may have figured it out – I had a fairly complex shape (you can see on the side of the screen shot above) and wanted to trim everything outside the rectangle.
After some further experimentation, it appears my use case was too complex for boolean operations, since I was able to get the boolean assistant when I selected just 2 rectangles off to the side.
I’ll do the trimming in the original illustrator file, unless there is some other way to skin this cat. Appreciate any recommendations.
Tried the scissor tool, and it does work. The scissoring was a little too tedious due to the shape, so I used the shapebuilder tool in Illustrator to cut down the shape to what I needed. Thanks for the suggestion.