All the Data Names by default will be visible to all the clients.
However, while creating multiple Data names (Data Location names) using Tally.Server Monitor, you can hide the specific Data Name by using a @@ symbol before the path. If data names are hidden, during select company etc. they will not be listed. However if the user knows the DATA LOCATION NAME by typing it, all the companies in that location will appear for selection.
You can also restrict your clients to access the respective companies by using Security control of that specific company.