The setsizecvt command was designed to be as simple as possible; the packaging tools process the sets in much the same way they process packages.
Executing in a set's installation directory, setsizecvt collects the space file (if it exists) and the setsize file from each of the packages included in that set. The setsize file is a file whose entries are formatted as follows:
pkg:/path/name #blks #inodeswhere pkg is the short form of the package name, and the rest is the directory and number of blocks and inodes used in that directory. This setsize file is created when the sets are created. Typically, the setsize file for a given set would be created from the pkgmap files for all of the packages in that set.
setsizecvt selects those entries in the setsize file for packages (in the current set) that the user wants to install. Those entries are then collected in a new file called space.
pkgadd uses the space file to see if there is enough space on the disk to install the set. The space file for a set is treated the same way as it is in a package.