I am I right to understand that a client can save more money on Licenses by using NAV2013 web clients (as i assume it is not included in per user license) than implementing NAV 2009 where he has to buy user licenses.
The actual client (RTC vs. web interface) doesn't matter. It's what the user does that matters. In NAV 2013, any users that only need read access, or that will only write to/update 3 tables (in addition to some core ones that are excluded from the count) can use a cheaper "Limited User" license. It's about 1/5 the cost of a full user license, and is still concurrent usage-based. Limited Users can use either the RTC or the web interface. The same goes for full users.
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