A project was carried out at The National Institute of Standards and Technology (NIST) to study the feasibility of developing a new-generation, multi-room, compartment fire model computer code, called the Consolidated Compartment Fire Model (CCFM) computer code. The idea was that such a code would consolidate past progress in zone-type compartment fire modeling, and allow readily for integration of future advances with the greatest possible flexibility. The project lead to the development of a prototype multi-room CCFM product called CCFM.VENTS. This is Part II of a four-part report which documents the above effort. The main obejctive of this Part II document is to document the design and underlying structure om the CCFM.VENTS computer software. It serves as a guide for those persons interesting in expending, modifying and if necessary correcting CCFM.VENTS at a later date. The other three parts of this report are: Part I: Technical asis; Part III: Catalog of Algorithms and Subroutines and Part IV: User Reference Guide.