This is the level of information that can often be gathered by automated WMI scripts. It shows the current configuration (disk space, IP addresses, RAM etc) of all equipment connected to your network. If level 1 data is collected regularly and stored, it can also provide a history of changes.
In a nutshell: What we DO have.
This is typically a database but could be a large spreadsheet, containing a list of all the hardware and software services that are in use. It can also hold details of old systems that have been decommissioned. Software Licence details are also held here.
In a nutshell: What we SHOULD have.
A top level summary of what each system delivers: how many users connect to database X, how many GB of data are transferred by system Y. If a history of this is built up over time, you can spot systems that are overloaded (or under-utilised) before they become a problem.
In a nutshell: What is being USED.
A list of the planned new systems and upgrades, a roadmap of future hardware refreshes and software updates.
In a nutshell: What we WILL have.
With this documentation in place, it can prove useful to look for differences between the categories, for example comparing level 1 and level 2 could show that hardware has been lost or stolen or perhaps items have been surreptitiously added to the network. Differences between Level 2 and Level 3 will indicate when core infrastructure will need to be upgraded to deal with future demand.
ITIL Change/Release management
ISM Integrated Service Management®
Oracle Naming Conventions
“In business, the idea of measuring what you are doing, picking the measurements that count like customer satisfaction and performance... you thrive on that” ~ Bill Gates