According to Snorri Sturluson's work, Niflhel could be interpreted as the lowest level of Hel, but Niflhel and sometimes Hel are conflated with the concept of Niflheim, a term which originates with Sturluson.
Snorri Sturluson's account implies that Niflhel could be seen as the most inferior level of Hel, yet it is often conflated with the concept of Niflheim, which Sturluson himself created.