Hi Jang...
With FM, the content model is defined by the EDD (and DTD) of the selected structure application. For ease of use, the default DITA Topic structure application is based on the ditabase.dtd which includes all of the DITA domains. If you want to strictly focus on a specific domain (set of elements), you'd need to develop your own structure application that is based on the model you want to work with. If you do this, then you switch between these domains by selecting the associated structure application when opening a file.
Note that with DITA, the Topic, Map, and Book structure applications are also set in the DITA Options dialog. These apps are used when FM opens a DITA file through the course of automatically resolving a reference (topicref, conref, xref) since there is no opportunity to manually select the application to use. If you do develop domain-specific apps, you'll probably have a bit of extra work because you'll need to change these apps in Options as well.
If you just want to reduce the number of elements (or eliminate specific domains from the "Topic" content model), you can modify the EDD to eliminate these elements. This will make it so these elements are not exposed as being available when authoring in FM. If you do make modifications to the default EDD (or other application files) it is strongly recommended that you clone the apps to create new application names of your own. This way, *when* you mess something up (and you will .. we all do) you'll be able to refer back to a fully functional app.
Cheers,
...scott