The ASHRAE Duct Fitting Database (DFDB) is a cloud-based application, rather than a single document, containing loss coefficients for over 200 fitting types. The 2016-era data and methodology are primarily documented within the Duct Design chapter of the ASHRAE Handbook. Access the current database through Duct Fitting Database - ASHRAE
Without the 2016 database, you might have wrongly used (C = 1.5) (old data), overestimating loss by 25%, leading to an oversized fan and wasted energy. ashrae duct fitting database 2016