About "fix atom/swap"

Hi all
I’m not clear about the “fix atom/swap” atomic type swap that follows this command. For example, we select Type 1 and Type 2. So in progress 1 and 2 swapped with each other.?Or 1 and 2 exchange with other atoms? I would appreciate it if you solve my problem

The documentation is very clear on this:

Swap candidates must be in the fix group, must be in the region (if specified), and must be of one of the listed types. Swaps are attempted between candidates that are chosen randomly with equal probability among the candidate atoms. Swaps are not attempted between atoms of the same type since nothing would happen.