Excellent explanation thanks.
quoted from website:
"An Ethernet segment identifier (ESI) allows a CE device to be multihomed to two or more PE devices—in single/active or active/active mode. PE devices that are connected to the same Ethernet segment discover each other through the ESI."
I saw another post where a guy had a subtended ethernet switching network composing of 5 switches, and the key point was that it was the same broadcast domain, thus he required the same ESI as he was experiencing broadcast storms.