Roman Shieldwall or testudo?

[Shield And Spear Art]

Robert Vermaat: The testudo was not exactly a battlefield formation but more of use in sieges.

Cassius Dio talks about Crassius not "locking shields" against the Parthians because of the tactics they were using, (book 20.)  I interpret this as something the Romans would do under archer fire in certain cases in open battles where they were not in danger of being charged by cavalry or fast moving forces, but that's my take.