What is the meaning of hbcolid field?

Hi,

I just connect to SQL Server and invoke the following
command:

Select * from sys.sysrscols

The result table contain a field ‘hbcolid’, which seem to be
identical to ‘rscolid’. I think rscolid is surely for rowset column id. Then
what is the meaning and usage of hbcolid? Is there any document on this?

Thanks