currently we use custom attributes named like "application". We use the values of this attributes to show the virtual machines that are representing the same applications within the business view. In this way, we see alarms and resources of the application. We also want to organize VMs about "using through end users". Here we use another cutom attribute. Etc.
After we have made the upgrade to the newest vCenter version, we use custom attributes. At the moment we didn'tmigrate to categories and tags. VMware will prefer the new web client and then we have to use categories and tags. You didn't see custom attributes in the web client.
I hope, that my explanation helps you.