Arrays in assemblies

[un1corn]

OS: Windows 10
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.18.16110 (Git)
Build type: Release
Branch: (HEAD detached at upstream/releases/FreeCAD-0-18)
Hash: f7dccfaa909e5b9da26bf50c4a22ccca9bb10c40
Python version: 3.6.6
Qt version: 5.6.2
Coin version: 4.0.0a
OCC version: 7.3.0
Locale: English/UnitedKingdom (en_GB)

Hi everybody.
I have been using freecad for some while now and tried something I hadn't previously.
I have created a small quick file to illustrate my issue.
In the attached capture you will see an array of 6 cubes, I wanted then to attach the round disc to the top of the origin cube of the array and create another array of the disc, but it would not connect and generated an error as shown.
Is there a way to achieve this or should I create each array as a separate file and import into an assembly file ?
Another question if I may, In capture 2, I have indicated the subassembly area in the import section, can someone point me to the text showing what this does and how it is used?
Thanks in advance guys.