WARNING: IF YOU **** UP, DESTROY, ETC YOUR NOKIA INTERNET TABLET, MMC/SD CARDS BY TRYING/USING THIS SOFTWARE, I WONT GIVE A ****. OR TAKE RESPONSIBILITY. This is provided 'as is', any damages while installing, using etc are solely your fault i.e I will not incur any blame. Saying that, I will try and help you out.
mmc-unionfs extends your internal file system's available space (both from the user's and the programmer's perspective) by up to the total amount of your MMC. That means that, after having installed it, you can use your MMC for anything (only exception: low level firmware) you can do in internal memory without any modification to the applications or the file system (like links pointing into the MMC).