An Erlang measures load on a Circuit-Switched link. Quoting Russ Rowlett's page:
The erlang is a dimensionless "unit" representing a traffic density of one call-second per second (or one call-hour per hour, etc.).
The classical definition of an Erlang was developed in the early 1900s by Professor A.K. Erlang. Thus, his definition does not apply generically to data traffic, because there is no standard definition of a "call" in data traffic, nor is there call-blocking as you would find in a fully-utilized Circuit-Switched link.
Edit for the OP edit:
Erlang-B and Erlang-C evolved from classical analysis of circuit-switched networks, but they can be adapted for use in data networks