What are the practical consequences of a metapackage?

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metapackages allow you to group a set of functionality under a convenient name for easy installation. They are empty packages which only have dependencies. They are only really helpful for distribution or released packages. They are especially designed for binary based installations.

If you are installing from source the grouping can only be done by the repository higherarchy in svn. (In DVCSs you can usually not even checkout subtrees and are limited to the repos.

I would recommend that you look into the rosinstall_generator for your purposes of checking out arbitrary code. But again it will work much better for released code.

For arbitrary code subsets making common rosinstall file snippets for rosinstall or wstool are probably your best approach.