You can use the Arduino IDE tabs in two ways (at least).
You can name the files in each tab as NNNN.ino and the compiler will include them automatically in alphabetical order after the principal .ino file (the one with the same name as the Directory). As far as I can see later files can access global variables defined in earlier files.
Or you can name the files NNN.h and NNN.cpp (or anything else that takes you fancy as far as I can see) and then use #include "NNN.h" in the principal file to include them. This has the advantage that you can control the order of inclusion of the files.
...R