1. What are excluded files and how can I tell which files are currently included or excluded
2. How can I exclude or include files in Simplicity Studio (and the legacy Precision32 IDE)
1. Projects can be configured to optionally ignore certain files when building, but leave the file present in the project directory. These files are greyed out and have a marked icon indicating that they will be excluded.
Simplicity Studio project with rtcdriver.c excluded
2. In Simplicity studio, files can be included or excluded by right clicking on the file in the project explorer and selecting the Resource Configurations->Exclude from build option. A menu will appear and allow you to configure which compilers include or exclude the file.
Alternatively you can navigate to the file's properties menu and toggle the Exclude resource from build option in the C/C++ build properties.
Lastly, each project can specify filters that cause files matching a pattern to be ignored/included.
To access the list, right-click on the project and select Properties. Then go to Resource->Resource Filters. Here, you can edit or add filters. There are numerous ways you can construct your filter, and regular expressions are supported.
For more information on regular expressions, there is a nice interactive tutorial here.