This setting is the root directory under which all network shares are expected to be manually mounted - locally on the Webinator machine - by the administrator, i.e. for file:// URL walks. For example, if the value is set to /mnt/local, then the share MyHost/MyShare should be mounted on /mnt/local/MyHost/MyShare, for walking file://MyHost/MyShare/....
This setting is ignored for profiles where Network Share Access Method (here) is available, Current, and in effect; see details there. This setting is also ignored for profiles where Proxy (here) is set, as the proxy is used instead. It also will not appear at all if Webinator is running under Windows (where UNC paths are used), or is not licensed for file:// URLs.