There is an explanation of the ESAC code at Beyond Midi - The Handbook of Music Codes.

Here's what it says:

The Essen Associative Code (EsAC), developed by Helmut Schaffrath, has been the backbone of a series of projects in the transcription and analysis of folksong repertories. More than 14,000 works have been encoded in a database framework with six basic fields that supports twelve basic search types aimed at identifying musical similarity. Plaine and Easie Code, developed in the Sixties by Barry Brook and Murray Gould, has been the basis of the musical incipit databases of the Répertoire International des Sources Musicales (RISM). Almost 300,000 incipits have now been transcribed in databases containing more than 100 fields. These materials are used both to catalogue works and to locate matching and derivative versions of works in multiple locations.