This is the place where intrepretations of laws are made by BCIS to lay out the procedures affecting various visas. http://www.immigration.gov/graphics/lawsregs/handbook/chronpol.htm
For example a June 2001 memo in this section talks about the H1 portability laws
http://www.immigration.gov/graphics/lawsregs/handbook/ac21guide.pdf
The latest memo on AC21 law was released as recent as august 2003...
http://www.immigration.gov/graphics/lawsregs/handbook/I140_AC21_8403.pdf
For example a June 2001 memo in this section talks about the H1 portability laws
http://www.immigration.gov/graphics/lawsregs/handbook/ac21guide.pdf
The latest memo on AC21 law was released as recent as august 2003...
http://www.immigration.gov/graphics/lawsregs/handbook/I140_AC21_8403.pdf