A clausal implicature is a quantity implicature which is inferred by an addressee concerning the truth of a proposition expressed in a particular subordinate or coordinate clause. The addressee infers that the proposition may or may not be true.
The complex or compound sentence of which the clause is a part does not indicate whether the proposition expressed by the clause is true or false.
There will be another sentence available which would entail that the proposition expressed by the clause is true. This sentence must be identical to the sentence under scrutiny except that, in one of its clauses, an expression has been substituted which is stronger than that used by the speaker.
- The sentence I believe that John is away implies that it is possible that John is in fact not away.
If John were certain to be away, then the expression would have been something like I know John is away.