Question
What type of salt should be used for this?  Table salt or rock salt?

Answer
Hi Louise;
I don't use salt to do this.
Actually I have never used anything to remove all vegetation forever.
I am sure after a time the salt is going to wash out of the soil, and vegetation will again start to grow.
10% acid white vinegar will remove vegetation effectly, but it will have to be used more than just once, probably every year, when anything starts to grow.
If you kill all the vegetation with vinegar, and then put down a coverage to prevent it from growing back, it will be mostly effective.
When I put down landscaping cloth to keep grass and weeds from growing in an area, and put gravel or bark mulch on top of it, 4 inches thick, after a time, dirt blowing and settling into that area will be enough to sustain growth, so shallow rooted vegetation will start to grow on it.
The only way I know for sure to keep it from growing back at all, is to pave over it.
Charlotte