I'd say that a flat bow is a generic term and that an ALB is a style of flat bow. The ALB is a classic design of the 1940's and were typically lemonwood, osage, yew and hickory.
Comparing an ALB and ELB the ALB has a flat, rectangular cross section where as an ELB has a deep, rounded belly. On a side note, some ELBs, made with less compression strong woods like elm, ash, etc. were more flat than rounded because of that less compression strong wood used.