When I moved an app to the Storage Card on my HTC One V, I found some data moved to USB Storage App. The Storage Card is still showing not used. What's "USB storage app"? Why apps are not moving into the Storage Card, instead moving into the "USB storage app"?
I guess you have to read the "USB Storage App" as the USB storage used by the app itself. So the code of the application.
If your application would for instance start generating cache information, saved pictures etcetera. And if it would save that information on your Storage Card, then the information will be showed under Storage Card.
So under "App" and "Storage Card App" is only the information and code of the application itself, not the information or data it generates. So in most case these 2 numbers will not change over time. And then under "Data" and "Storage Card" information generated by the application can be found. Either on the Storage Card or the internal memory (depending on the application I guess). This is the information managed by the application (local databases etc), so this will probably change over time, depending on the application in question of course.
This is my interpretation of those 4 terms. And if I check for some applications I use, it looks like my interpretation is correct. It is certainly correct for the applications I created myself, so for these apps I know where what information is saved on the phone.
The answer posted by @ndsmyter seems almost correct. The applications under
Application manager -> SD CARD are those applications which can, at least in part, be moved to the SD card (and this happens where the checkbox next to the application's entry is checked). At first sight it may seem like "Application" and "SD card app" are for code on internal storage and on external storage, respectively, whereas "Data" and "SD card data" are for application data generated by the application on internal storage, and external storage, respectively.
However, upon further inspection I realized that on my Samsung Galaxy SIII I don't have an external SD card inserted, and despite this, while none of the applications listed under
Application manager -> SD CARD have the
SD card app field displaying anything but
0.00B, the example below, and many others, shows that
SD card data is nonzero even though I don't have an SD card (click the image to enlarge):
So, the only thing I am able to conclude is that these values are not the number of bytes taken up by the application and its data on internal and external storage, but rather how the data would be distributed among these four locations should the user decide to transfer the application to the external SD card (and some portions would still need to reside on internal storage for technical reasons).
Also note that on my device the field names are different. What on the original poster's post is called USB storage app on my system is called SD card app, very confusing.
And just as confusing is the fact that when I change the UI language to Italian, what in English was called "SD card app" now becomes "SD card applications" (applications: plural), which means not even the people that localized the system could figure out what these fields represented.
"USB storage" is the "internal" SD card i.e. the partition on the phone's internal memory that is treated like an SD card for general data storage.
"Storage card" is your actual physical microSD card.
The latter is where the stuff moved via "move to SD" or "move to storage card" gets put.
As for why it does this, Android's system for storing app stuff in user storage hasn't caught up with the concept of having more than one section of user storage.
It seems to be the .android_secure folder on your SD card.