The method, type, or assembly has no concept of a CER. It does not take advantage of CER guarantees. This implies the following:
In the face of exceptional conditions the method might fail.
The method might or might not report that it failed (it is non-deterministic).
The method is not written with CERs in mind (which is the most likely scenario).
If a method, type, or assembly is not explicitly marked to succeed, it is implicitly marked as Cer.None.