Here is some more information that I found recently.
What is a Class II Transformer?
A Class II transformer is used to supply Class II circuits. Class II transformers have a maximum VA (Volt-Ampere) rating of less than 100 and a maximum secondary output of 30 VAC. The maximum VA generally offered is 75 and the most common secondary voltage is 24 VAC. All Class II transformers are either inherently or non-inherently limited. This means that the maximum output current of the transformer is limited, either by the intrinsic coil impedance or by a fuse or circuit breaker. These transformers are designed to meet the requirements of U.L. 1585.
Inherently Energy Limited Transformers - Class II transformers up to 50 VA are "Inherently Limited" which means that the transformer, if overloaded, will short itself out and fail safely, not requiring a fuse.
Non-Inherently Energy Limited Transformers - 60 thru 75 VA Class II transformers are generally protected by a resettable circuit breaker or a fuse within the transformer secondary. Without this overload protection, the transformer would not satisfy the safety requirements for a Class II circuit.