The reserve command was added to facilitate the move from CASTOR to CTA. It just sets the current max id value for files and containers(directories) to the given values. This means that once you create a new file/container(directory) in your instance it will get an id which is greater then the values you set. This should only be used once in the beginning once you set up the instance and you want your new ids to be greater than a given value. In normal circumstances there is no clash of file ids, but if you import some data and inject in the eos namespace and you want to have a particular file/container id for the imported file then you can do it - and to avoid any conflicts you might want to use the eos ns reserver-ids if you now the max id value that you are about to inject. Hope this helps!