Just to add a note to JP's good info. When DLU's are completely exausted things will run just fine but you will not be able to add any "new" devices. One thing to keep in mind is that most systems have a slight License overdraft (so you may have some grace here);
The License "Overdraft" is determined by Cisco and can be between 5 and 15%. It will show on the License File. Have a look (at this line in the file);
OVERDRAFT=50 indicates (5% of 1000) allowed overdraft. Cisco determines the overdraft value.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.