The difference, like the other commenter already pointed out, is whether you take something across a border, and then leave it there.

If you take the stuff you brought with you, back when you leave, then you didn’t import it. Import tax is exactly that, a tax on stuff moving from one country, to another.

Personal items aren’t subject to this, because the owner is coming and leaving with them. Technically there’s a whole song and dance that should happen when you come and go, but that’s massively inefficient, so customs will just sit there and trust that if you have something to declare, you will.