It goes further than that, I think.

Durham is not allowed to argue that the collusion claim was false.

He can only argue that specific points made Sussman were false, and he can present evidence that proves the lie and that Sussman knowingly lied.

This is largely what I expected. Contrary to the interpretation of Q's stance about a small trial having enormous impact through tangential evidence being revealed, most court systems will keep the evidence very focused and their rulings narrow.

Durham is only allowed to present arguments and evidence that counters Sussman's specific claims, and nothing more. If he wants to prove that the collusion claim was a conspiratorial lie, then he won't be allowed to do it during the Sussman trial.

It goes further than that, I think.

Durham is not allowed to argue that the collusion claim was false.

He can only argue specific points that Sussman made that were false.

This is largely what I expected. Contrary to the interpretation of Q's stance about a small trial having enormous impact through tangential evidence being revealed, that's not how most court systems work.

Durham is only allowed to present arguments and evidence that counters Sussman's specific claims, and nothing more. If Durham wants to prove that the collusion claim was a conspiratorial lie, then he won't be allowed to do it during the Sussman trial.