If you are a little bit confused or have read something else (we have found some misleading information on our research for this grammar).

1) the conjunctive I is exclusively used for reported speech. It does not have any other function!
2) the conjunctive II appears in the reported speech under a.m. conditions.

There is no difference in the meaning between conjunctive I and II.

Example:

Er sagt, er gehe in die Schule. (conjunctive I) Er sagt, er ginge in die Schule. (conjunctive II)

Both sentences mean the same. The fact that conjunctive II is used even if the conjunctive I is not ambiguous is only related to the fact that the living language is not rule based anymore - be it that the speaker does not know the rule