regexps.com
You can define 
meta-projects
 which are combinations of individual
projects that are separately tracked by arch
.  This allows you to
divide a large project into smaller, more manageable pieces, each of
which can develop independently of the others, and each of which can
be a part of more than one meta-project.
This is accomplished by writing config specs , which define the contents of the meta-project and how they should be arranged in a source tree.
For example, arch
 itself is a meta-project.  The source tree
contains:
        dists/
          dists/src/
            dists/src/arch/
            dists/src/file-utils/
            dists/src/ftp-utils/
            dists/src/hackerlab/
            dists/src/shell-utils/
Each of those directories is the root of a project tree (contains a subdirectory named {arch} ).
The topmost directory, dists
 also contains a subdirectory named
configs
.  In that subdirectory are the meta-project configuration
files.  For example:
        dists/
          dists/configs/
            dists/configs/regexps.com/  # Tom's configuration files
              dists/configs/regexps.com/devo.arch
              dists/configs/regexps.com/release-template.arch
Here are the contents of devo.arch
:
     # 
     # Check out an arch distribution from the devo branches.  
     # Latest revisions.
     #
     ./src                   lord@regexps.com--2002/package-framework--devo
     ./src/arch              lord@regexps.com--2002/arch--devo
     ./src/file-utils        lord@regexps.com--2002/file-utils--devo
     ./src/ftp-utils         lord@regexps.com--2002/ftp-utils--devo
     ./src/hackerlab         lord@regexps.com--2002/hackerlab--devo
     ./src/shell-utils       lord@regexps.com--2002/shell-utils--devo
     ./src/text-utils        lord@regexps.com--2002/text-utils--devo
Each (non-blank, non-comment) line in that file has the format:
        LOCATION             CONTENTS
which means, to create the meta-project, get the revision indicated by
CONTENTS
 and install it at LOCATION
.  The CONTENTS
 field can be
a branch (meaning, get the latest revision of the latest version on
that branch), a version (meaning get the latest revision in that
version), or a revision name (meaning get that revision, exactly).
To check out an entire arch
 tree, I first check out dists
 from
devo
, then use build-config
:
        % tla get dists--devo dists
        [....]
        % cd dists
        % tla build-config regexps.com/dists.devo
        [....]
Once you have a meta-project tree, some other useful commands are:
     cat-config : output information about a multi-project config
One use of that command is to generate a list of sub-projects to which some other command can be iteratively applied:
        % tla cat-config CFGNAME | awk '{print $1}' | xargs ...
Additionally, the option --snap
 can be usefully applied to a
configuration that names subproces by version rather than revision.
It examines the project tree to see what revisions are actually
installed at each of the LOCATIONs
.  Then it writes a new config
which specify those REVISIONS
 precisely.  This is useful, for
example, for recording the specific revisions you are about to turn
into a distribution.
regexps.com