current nopCom's design is combine BLL and DAL together in Nop.Common proj, however, i don't think this is an good idea, since if i change my business logic, i have to compile DAL code, this is a not good practice of 3-tier architect. so i suggest split these 2 folder to 2 different project, in case we can replace BLL.dll only if we deploy new business logic, or replace DAL.dll only if we deploy new DB changes.