subdir is an option that may be specified in any of the sections of the ConfigurationFile. It is always appended to the RootDirectory of a given project.

It by default is empty (or "."), to indicate exactly the same filesystem place pointed by the RootDirectory.

You can use it for example to relocate the upstream content giving it a particular prefix:

[pyapache]
source = cvs:pyapache
target = darcs:pyapache

[darcs:pyapache]

[cvs:pyapache]
repository = :pserver:anonymous@cvs.sf.net/pyapache
module = pyapache
subdir = PyApache

that will add a prefix of PyApache/ to every path in the SourceRepository

When the two repositories specify different pathnames tailor uses DisjunctWorkingDirectories.