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3rd chunk of `chattr.man`
256bde0aa3771719f00e8136e0dbb0f67f6221baa23d6f68000000010000092c
 per‐file compression.

       N      A file with the ’N’ attribute set indicates that the file has data stored inline, within the inode itself. It may not be set or cleared using chattr(1), although it can be displayed by lsattr(1).

       P      A directory with the ’P’ attribute set will enforce a hierarchical structure for project id’s.  This means that files and directories created in the directory will inherit the project id of the  directory,  re‐
              name  operations  are  constrained so when a file or directory is moved into another directory, that the project ids must match.  In addition, a hard link to file can only be created when the project id for the
              file and the destination directory match.

       s      When a file with the ’s’ attribute set is deleted, its blocks are zeroed and written back to the disk.  Note: please make sure to read the bugs and limitations section at the end of this document.

       S      When a file with the ’S’ attribute set is modified, the changes are written synchronously to the disk; this is equivalent to the ’sync’ mount option applied to a subset of the files.

       t      A file with the ’t’ attribute will not have a partial block fragment at the end of the file merged with other files (for those file systems which support tail‐merging).  This is necessary for applications  such
              as LILO which read the file system directly, and which don’t understand tail‐merged files.  Note: As of this writing, the ext2, ext3, and ext4 file systems do not support tail‐merging.

       T      A directory with the ’T’ attribute will be deemed to be the top of directory hierarchies for the purposes of the Orlov block allocator.  This is a hint to the block allocator used by ext3 and ext4 that the sub‐
              directories  under  this  directory  are  not  related,  and  thus  should  be spread apart for allocation purposes.   For example it is a very good idea to set the ’T’ attribute on the /home directory, so that
              /home/john and /home/mary are placed into separate block groups.  For directories where this attribute is not set, the Orlov block allocator will try to group subdirectories closer together where possible.

       u      When a file with the ’u’ attribute
Title: chattr Attributes: N through u
Summary
This section describes additional attributes that can be set/viewed using `chattr` and `lsattr`. It explains the functions of 'N' (inline data), 'P' (project ID hierarchy enforcement), 's' (secure deletion with zeroing), 'S' (synchronous updates), 't' (no tail-merging), and 'T' (top of directory hierarchy for block allocation in ext3/4). It also mentions the 'u' attribute, without detailing its function.