What is the purpose of "Tag0" and "Tag1" in the taint widget?

Tag0 and Tag1 in the taint widget indicate which data should be marked. Having 2 tags allow you to mark differently some pieces of data, for example:

Tag0: rax
Tag1: [0x123456; 8], rbx

This will follow the data inside rax and mark with Tag0 all of the locations that it goes through, and it will follow the data inside [0x123456; 8] and rbx and mark with Tag1 all of the locations that it goes through. This allows to know if any tainted data comes from rax or rbx/[0x123456; 8].

You can see the results affecting each tag in the corresponding tab of the taint widget.

For more information, you can refer to the documentation on the taint widget.