Essentially it's a method of Licensing per user rather than per device and it opens up the options to easily add other UC applications besides dial tone. There are now four different levels of CUWL. Business Edition, Entry Edition, Standard Edition and Professional Edition. The following document has a table detailing what comes with each.
So if you were to purchase, for example, a CUWL standard licence for each of your users they would receive
Call Control for one Cisco IP Phone
Messaging (eg Unity or Unity Connection)
Soft Client (either IP Communicator or Personal Communicator)
Mobility (Single Number Reach)
The professional level provides for pretty much the full suite of Cisco UC offerings.
A few important things to remember is that 3 years of UCSS is mandatory unless your a government or education organisation where 1 year is required, CUWL on CUCM requires a minimum of 100 users, and you can't mix the two licensing models (ie CUWL and a la carte DLUs) though you can mix the CUWL editions, eg 20 Professional and 100 Standard.
In addition to the information provided by Paul, please bare in mind, that you can not have Al-a-Cart and CUWL mixed in your setup.
So today if you decide to go for Standard CUWL and tomorrow if you want to add MeetingPlace Express (10 ports only), you'll have to upgrade to professional, becuase you can not purchase MPE al-a-carte.
So you should be very clear what you want to have today and in near future.
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