Lets first understand the ARQ a bit, data transmission utilizes various forms of Automatic Repeat reQuest (ARQ), where data which has not been received or correctly decoded is automatically requested. This guarantees delivery of the full packet of data, for instance to make sure that the whole web page is displayed. When data is known not to have been received, there normally follows several further attempts to send/receive the data in the same format.
Usually this will be sufficient to ensure the data gets through, however if the data is still not received and decoded correctly even after several repeat attempts, it could be that more protection needs to be added to the basic data or for it to be formatted in a different manner. LTE utilizes a process known as re-segmentation, which essentially allows blocks of previously formatted data to be segmented further. The data can then be sent in smaller packets with added protection, hopefully giving the data a better chance of being received and decoded correctly.
I hope this clarifies a bit, by the way re-segmentation is used only for AM mode.