Hi KhoeHuynh,

re:
> But is there any way I can cut just the top part?

If you select the top face as the projection target then it will only make a projection there.

You can select face or edge subobjects on a solid by a second click on it. The first click will select the whole solid, and then when the whole solid is selected a second click on it will "drill in" to select faces or edges.

So in your case where you want to project a line onto the top face, select the line:



Then run Construct > Curve > Project.

The prompt in the upper right corner of the window will say "Select projection target".

Select the top face by 2 clicks on the box, the first click will select the whole box and the second click on it will drill in to select just faces or edges:



Then click "Done" or right click in a viewport (shortcut for "Done") and you'll be at the last step in the project command where you can adjust the projection direction. Click "Done" or right click in a viewport again to finish the Project command with this result:



Note that if you want to actually cut the whole box into 2 separate solid chunks you can use boolean difference to do that without needing a separate projection step and if you want to cut just the top face into 2 separate faces you can use Edit > Trim to do that without needing a separate projection step either.

- Michael