Have you had a read through at https://developer.bigfix.com/ yet? This is the go-to resource for custom content, explanations of client relevance vs session relevance, best practices, etc.
“Client Settings”, as I think you’ve found, are those things you can modify in the “Edit Computer Settings” dialog. They are stored on the client - in the Registry for Windows, or in besclient.config for Linux/UNIX systems. You can create custom settings via tasks/fixlets.
“Retrieved Properties” consist of anything you read from the client - file contents, registry entries, WMI, DMI, etc. You can create Global Properties or create an Analysis to retrieve properties. Retrieved Property consist of some client relevance that is evaluated at the endpoint, and the results are reported up to the server. “AIX OS Full Level” is an example of a Retrieved Property. You can use the “Manage Properties” menu item from the console to determine whether this is a Global Property, or provided by some Analysis. You can also use the “Manage Properties” menu to read the relevance that the client evaluates to determine the result, to see how the “AIX OS Full Level” value is determined.
In most cases you should avoid trying to evaluate things like “current site” at the client. There are some complex nuances to how that works, really not a good place to start learning. The “site” and “fixlet” inspectors in particular can behave differently depending upon whether you are using them in analysis/fixlet relevance, vs using them in an actionscript substitution; best to just avoid them at the start.
Likewise anything you read on objects in the “bes” family - “bes client”, “bes site”, “bes property”, etc. - all indicate the use of Session Relevance. Session Relevance evaluates on the Server / Web Reports, not the client. Session Relevance allows for powerful customizations such as reports, custom dashboards, and API integrations, but again is not the best way to start learning.
Have you come across the Fixlet Debugger? What OS flavors are you primarily interested in managing?