allow constraints will evaluate direct dependencies for a package. If any local dependency does not have tags that match a selector within the allow array, then the constraint will fail. allow constraints are used to explicitly define which dependencies a package should have. They are helpful for defining a hierarchy of packages or creating workflows.
Unlike disallow constraints, allow constraints only apply to direct dependencies.

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Allow specific direct dependencies

Use a tag as a selector to allow subsets of local dependencies. 
{
  "disallow": ["api", "internal"]
}

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Allow all dependencies

Use * as a selector to allow all local dependencies.
This is equivalent to not having a constraint at all but is useful when you want to be explicit and have the constraint show up in Commonality Studio or the CLI.
{
  "disallow": "*"
}