Bec is a thin layer around Bosch Sensortec's BSEC 2.x library (BSEC 1 may be used too). It's purpose is to allow easier integration in a C based code base, compared to using the C++ layer provided by Bosch Sensortec.
BSEC (Bosch Software Environmental Cluster) is a closed-source library; it is a necessary piece of software if values derived from the BME68x sensor's main measurement values shall be obtained, like IAQ, sIAQ, bVOCeq, or CO2eq.
Bec is decoupled from the actual BME68x driver and device; subdirectory bme provides support for the official BME68x Sensor API.
Only a single sensor instance is supported now, but multi-instance support may be added in the future.