Shell item -
The Windows Shell uses Shell Items (or Shell Item list) to identify items within the Windows Folder Hierarchy. A Shell Item list is much like a "path", and is unique to its parent folder. The format of the Shell Item is undocumented and varies between Windows versions.
The Shell Item is used in Windows Shortcut (lnk) file and the ShellBags key in the Windows Registry.
Format
The basic format is a list, consisting of a (shell item) entry size value (field) and entry data.
There are multiple types of entries to specify different parts of the "path":
volume
network share
file and directory
URI
Some shell item entries contain date and time values which can be used in Timeline Analysis.
Notes
From https://learn.microsoft.com/en-us/windows/win32/shell/nse-implement#interpreting-pidls
Some methods, such as IShellFolder::GetUIObjectOf, use PIDLs that are relative to the parent folder and are straightforward to interpret. ... What complicates the task of associating an object with a fully qualified PIDL is that one or more of the initial SHITEMID structures in the PIDL might belong to objects that lie outside your extension in the Shell namespace. You have no way of interpreting the meaning of the abID member of those structures. What your extension must do is to "walk" the list of SHITEMID structures, until you reach the structure that corresponds to your root folder. From then on, you will know how to interpret the information in the SHITEMID structures. 
Example
An example of a shell item list taken from Calculator.lnk
shell item type : 0x1f shell item sort order : 0x50 shell item folder identifier : 20d04fe0-3aea-1069-a2d8-08002b30309d shell item folder name : My Computer shell item type : 0x2f shell item volume name : C:\ shell item type : 0x31 shell item file size : 0 shell item modification time : Dec 31, 2010 13:28:48 UTC shell item file attribute flags : 0x0010 Is directory (FILE_ATTRIBUTE_DIRECTORY) shell item short name : WINDOWS shell item extension size : 38 shell item extension version : 3 shell item creation time : Dec 31, 2010 13:26:18 UTC shell item access time : Dec 31, 2010 13:28:52 UTC shell item long name : WINDOWS shell item type : 0x31 shell item file size : 0 shell item modification time : Dec 31, 2010 13:28:38 UTC shell item file attribute flags : 0x0010 Is directory (FILE_ATTRIBUTE_DIRECTORY) shell item short name : system32 shell item extension size : 40 shell item extension version : 3 shell item creation time : Dec 31, 2010 13:26:18 UTC shell item access time : Dec 31, 2010 13:28:38 UTC shell item long name : system32 shell item type : 0x32 shell item file size : 115712 shell item modification time : Mar 25, 2003 12:00:00 UTC shell item file attribute flags : 0x0020 Should be archived (FILE_ATTRIBUTE_ARCHIVE) shell item short name : calc.exe shell item extension size : 40 shell item extension version : 3 shell item creation time : Dec 31, 2010 13:06:06 UTC shell item access time : Dec 31, 2010 13:06:06 UTC shell item long name : calc.exe 
See Also
Jump Lists
LNK
External Links
MSDN: Introduction to the Shell Namespace (Windows)
Implementing the Basic Folder Object Interfaces
ShellBags Registry Forensics, by johnmccash, October 2008
Using shellbag information to reconstruct user activities, by Yuandong Zhu, Pavel Gladyshev, Joshua James, 2009
Windows Shell Item format, by the libfwsi project, July 2010 (work in progress)
Computer Forensic Artifacts: Windows 7 Shellbags, Chad Tilbury, July 5, 2011
MoVP 3.2 Shellbags in Memory, SetRegTime, and TrueCrypt Volumes, by Jamie Levy, September 2012
Shellbag Analysis, Revisited...Some Testing, by Harlan Carvey, October 2012
Shellbags Forensics: Addressing a Misconception (interpretation, step-by-step testing, new findings, and more), by Dan Pullega, December 4, 2013 (RESTRICTED)
Part 5: USB Device Research – Directory Traversal Artifacts (Shell bagMRU Entries), by Nicole Ibrahim, December 31, 2013
ReactOS: Shell Documentation