1) Device memory is actually not memory, it's a portion of Xperia C's 4GB storage, used to store system and user apps and whatnots.
2) SD Card is your actual external SD card - Can be used to store data, files and some apps that support it.
3) Internal storage is another portion of Xperia C's 4GB storage, which is only used for files other than apps (like pics, downloaded files, etc.) - I believe this is only used when you have no external SD card inserted.
There's another portion in Xperia C's 4GB storage that is used by OS (not visible in Storage)
You can only install apps onto no. 1 & 2. (I think it's OS restriction)
Yes technically you're supposed to be able to repartition that 4GB storage to enlarge no. 1 and shrink no. 3 like other (mostly Chinese) Mediatek phones, but this is not possible in Xperia C due to locked bootloader. (Yes I've tried it and bricked the phone, though I was able to recover).
The only thing feasible to gain more space for apps is to use Link2SD (which requires root, which in turn voids your warranty), I know no other way without root.
Keep in mind that I'm in no way advising any of you to root your Xperia C, this is my backup phone after I lost my Note 3 on New Year's Eve, and seeing how indifferent Sony is towards budget phone's owners, I decided to void the warranty to root and fix the stupid issues that plague Xperia C myself, like ringtone always resets to default, what I did was just overwriting the default ringtone with my own custom ringtone.
I used to own Xperia Z last year, and I didn't know that Sony was this ignorant and indifferent until I used their budget phone. I still wish other Xperia C users would get the necessary fixes ASAP.
