Carbon Credit methodologies will spell out all the rules for a specific type of carbon credit project.
There are many, many methodologies out there. Many were defined as part of the Clean Development Mechanism (CDM) and some of these remain valid today.
Methodologies are constantly being updated. Some of the popular methodologies have been updated through more than 20 versions.
Methodologies are retired from time to time.
Methodologies tend to be supported by various tools. These tools will take a specific consideration (eg. financial additionality) and define the rules for that consideration. In this way, methodologies can point to one single tool that is much easier to update, rather than requiring updated to all methodologies.