More info: http://www.apiesoft.com/article/transfer-purchased-items-to-itunes.html If your iPhone/iPod/iPad is connected to its home iTunes library, you are able to transfer iTunes purchases from iPod, iPhone or iPad back to iTunes easily by sync.

1. How to Transfer Purchased Items to iTunes
http://www.apiesoft.com/article/transfer-purchased-items-to-itunes.h

2. Question 1: I was attempting to update my phone to iOS 5.1.1
when this message popped up: "There are purchased items on
the iPhone that have not been transferred to your iTunes library.
You should transfer these items to your iTunes library before
updating this iPhone. Are you sure you want to continue?"
Question 2: When I want to sync my iPhone in iTunes on my
new computer I will lose all of the apps that I purchased. Can
someone tell me how to sync it in a way (or some other
solution) so I can keep my purchased apps? I'm using the
same ID as on old Mac. I have been searching on the internet
for help, but I can't find a clear answer. Thanks in advance.

3. If your iPhone/iPod/iPad is connected to its home iTunes library,
you are able to transfer iTunes purchases from iPod, iPhone or
iPad back to iTunes easily by sync. But if you want to transfer
your items to other iTunes library for sharing, you need
authorize the computer first and then transfer purchases from
iPhone/iPad/iPod to computer.

4. Guide:
How to Transfer Purchased Items to iTunes Library
Step 1: Authorize the computer before connecting the
device.
Step 2: Connect your iPod, iPad or iPhone to the computer
with iTunes open. Note: Clicking "Erase and Sync" will
delete any existing content currently on your device. The
content will be replaced with whatever existing content is in
the current iTunes library your device is connected to.

5. Step 3: iTunes will begin copying any purchases made with
any account that the computer is authorized for. There are two
ways to sync your iTunes store purchases to computer.
1. After connecting your iPod or iPhone to the computer and its
icon appears in iTunes, you can choose Transfer Purchases
from iPod from the File menu.

7. 2. Control-click your iPod or iPhone in the iTunes
Source list, then choose Transfer Purchases from the
shortcut menu that appears.

8. However, this feature only works for items that were purchased
on the iTunes Store. Any items imported from audio CDs or
acquired from other sources will not copy from your iPod or
iPhone to the iTunes library.
Related Article:
How to Transfer Music from iPod to iTunes
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