Make clear how do files become available to the user

docs/tutorial/cvmfs/what-is-cvmfs.md

@@ -49,7 +49,24 @@ triggered when search paths are examined.
 In certain cases, the CernVM-FS has also been used to
 [distribute large *data* repositories](https://cvmfs.readthedocs.io/en/stable/cpt-large-scale.html).
 
+## How do files become available to the user?
+Files in CernVM-FS are organized in repositories, in the form of content addressable storage.
+For every repository, there is a dedicated server [( Stratum 0 )](../appendix/terminology.md#stratum0) that is its single authoritative source, responsible for publishing modifications of its contents.
+Information on how to create a new repository can be found in the [CernVM-FS documentation](https://cvmfs.readthedocs.io/en/stable/cpt-repo.html).
 
+Repository administrators should discourage users from accessing Stratum 0 directly, but use a network of public mirror servers instead.
+[Stratum 1](../appendix/terminology.md#stratum1), is a public replica server containing repositories in their entireties, configured to automatically remain consistent with the stratum 0.
+In cases where users do not have access on the public servers, site administrators can [create private Stratum 1 servers](../access/stratum1.md).
+
+On top of Stratum 1, site administrators are encouraged to [install a series of forward proxies](https://cvmfs.readthedocs.io/en/stable/cpt-replica.html#squid-configuration) that works as yet another caching layer.
+This will improve the cold cache performance (especially on sites with clusters of nodes) on the client side (see [performance analysis](../performance.md)), but also reduce the load of the Stratum 1 servers.
+
+<div align="center">
+<img src="../img/cvmfs_topo.png" alt="CernVM-FS Topology" width="100%"/></br>
+</div>
+
+To access a repository, the user (here the CernVM-FS client) specifies the proxy that will forward its requests, and a list of Stratum 1 servers.
+Stratum 1 servers will be sorted automatically based on their geographic proximity (see [relevant documentation](https://cvmfs.readthedocs.io/en/stable/cpt-configure.html#sct-geoapi)).
 ## Features
 
 ### On-demand downloading of files and metadata { #features-ondemand }