What is the use of data/read/scroll/clear permission in OpenDistro elastic-search?

The documentation page is 100% silent about all the permissions.

How do we know what each permission is meant to do? I am specifically interested in the data/read/scroll/clear permission

As you mentioned, the documentations do not have this information. Best bet would be to search through the codebase to get the corresponding action class and then deduce from that.

For the given action data/read/scroll/clear, this is what I found.

https://github.com/opensearch-project/OpenSearch/blob/0ba0e7cc26060f964fcbf6ee45bae53b3a9941d0/server/src/main/java/org/opensearch/action/search/ClearScrollAction.java#L40

The permission is for clearing the search contexts associated with specified scroll ids.