Meanings of defragmentation and when it is needed

There seem to be two meanings of defragmentation: - **Defragmentation** is a process that reduces the amount of fragmentation. Fragmentation occurs when the file system cannot or will not allocate enough contiguous space to store a complete file as a unit, but instead puts parts of it in gaps between other files. - When under Windows, if we try to release existing free space from an NTFS partition (to later create a new partition especially during dual boot installation of Ubuntu besides an exisiting Windows OS), we will have to use Windows tools to move all the files to one end of the partition, and leave free space at the other end of the partition as much as possible. I heard this is also called **defragmentation**. Alternatively, Linux tools such as gparted can release free space from an ntfs partition without **defragmentation** (in the sense of the previous paragraph, not in the sense of the first paragraph) the NTFS partition first. ### Questions: ### 1. I wonder if the two kinds of "defragmentation" above always happen together? 2. Does whether a file system (e.g. NTFS) needs defragmentation depend on - the OS under which it has been used (e.g. Windows or Linux), or - the file system type (e.g. NTFS) itself? 3. Are the answers to questions in 2 different for different meanings of defragmentation (as mentioned earlier)? For example, I heard that, - in Linux, defragmentation in the first sense isn't needed on an EXT4 partition, unless a partition is occupied more than 90%, because Linux always try to automatically defragment. - Releasing free space from a NTFS partition by using the Linux tool gparted doesn't need to do defragmentation on the NTFS partition in the second sense, and is it because Linux always automatically move all the files to one end of the partition as much as possible?