All package and software information stored in an item's pkginfo file is copied to Munki's Catalogs. Software catalogs are used to filter out which version of a package is made visible to specific clients. In this way, we can stage 'testing' and 'production' environments to ensure that a deployment workflow is properly tested before being pushed out to the users.
Using MunkiAdmin in the example below, Firefox-52.0.1 is currently deployed to the Production environment. A newer version, Firefox-52.0.2 is currently set to only deploy to the Testing environment.
Once the software is set to the proper catalog, the client machine will have access to it from its manifest. In the example below, a tester client is only receiving software from the testing catalog. A workstation in the Library is receiving software from the production catalog.
While version 52.0.1 is available on both catalogs, Munki will only install the newest version visible to the client.