Usually water in such multi – stories is provided from a single overhead common tank. Water is feeded to the house through shafts.
For example there is a three bedroom apartment having three washrooms and one kitchen where water is feeded through three shafts. One shaft may feed one bathroom each and the remaining shaft may feed the residual washroom and kitchen together.
Now if you want to provide soft water to the entire house then three small units of water softener are installed . ( one with each shaft ). An outlet from the line which is providing water to a particular washroom is taken and attached to the softener. Water from the outlet of the water softener is fed to the entire washroom. In such cases , the water softener unit is kept preferably in the balcony nearest to the shaft.