The AuthDigestDomain directive allows you to specify one or more URIs which are in the same protection space (i.e. use the same realm and username/password info). The specified URIs are prefixes, i.e. the client will assume that all URIs "below" these are also protected by the same username/password. The URIs may be either absolute URIs (i.e. inluding a scheme, host, port, etc) or relative URIs.
This directive should always be specified and contain at least the (set of) root URI(s) for this space. Omiting to do so will cause the client to send the Authorization header for every request sent to this server. Apart from increasing the size of the request, it may also have a detrimental effect on performance if "AuthDigestNcCheck" is on.
The URIs specified can also point to different servers, in which case clients (which understand this) will then share username/password info across multiple servers without prompting the user each time.