BASICtools has always used a version of the C pre-processor. That allows you to #include other files, and that has always been used for definitions of hardware registers. As programs grow it is good to break up files into manageable and often reusable pieces. A feature of our MakeItC has always allowed searching of all files included in a project. That feature now exists in the latest BASICtools.
So now when your program has more than 1 #include file. A Show Project option will appear in the Files menu.
Also a Search option in the Edit menu has been added that will search all files #include'd in your project.
And in all cases if you double click on a file in those display boxes, it will be opened into your Text Editor.