Hi Mike, basically booleans already have projection built into them, so if you want to do a boolean you can do it directly with the original unprojected 2D curve. Inside the boolean command that 2D curve will get extruded into a solid and then that solid will be what's booleaned.

When you have a projected curve, that won't work for a boolean anymore since a curve that's bending around in 3D space doesn't have a really well defined extrusion direction anymore unlike a 2D curve has with its plane normal.

It is possible to use a projected curve in other ways though, like you can use it as a path for sweeps and also you can use it as a cutting object in the Edit > Trim command which works at a surface level instead of at a solid level. Trim also has projection built into it as well though so you can use it directly with the 2D curve as well instead of doing a separate projection step.

If you have more questions about the projection or how to do the boolean could you please post your 3DM model file with your objects in it? That would help to give you more specific advice.

- Michael