Every Post canonical system can be reduced to a string rewriting system (semi-Thue system). [...]
It has been proved that any Post canonical system is reducible to such a substitution system, which, as a formal grammar, is also called a phrase-structure grammar, or a type-0 grammar in the Chomsky hierarchy.
And on a different page it says:
A semi-Thue system is also a special type of Post canonical system, but every Post canonical system can also be reduced to an SRS.
I have not been able to find a good explanation of these statements.
Is there a good reference that shows how to do conversions between these notions?