Hi Eddi, yeah I think Jonah has it described well above.

If you use Edit/Separate on that object, you should see that there are 2 pieces there that you can drag away independent from one another.

A seam edge is when there is the same 1 surface on both sides of the edge.

It is possible for edges to change their configuration after things have been sliced up in booleans like in your case there.

- Michael