A LDCad shadow library is a collection of LDraw (part) files organized identical to an official LDraw library release. These files can be placed in ether a folder or be compressed into a zip file renamed to .csl.
When a shadow library is assigned to a library location (using the Prefs/LDraw search locations dialog) content inside it will be loaded whenever a corresponding library item is loaded.
For example if 3001.dat is loaded from the official library's parts folder LDCad will check if there's also a 3001.dat inside the shadow location's parts folder, if so it will also load that file. Next it will add any useful information (like !LDCAD snap metas) to the (in memory) official library 3001.dat data.
Shadow libraries are currently exclusively used to add part snapping information to a growing subset of official parts using the default offLibShadow.csl shadow library file.
Editing a shadow library
If you want to edit the default shadow library (e.g. to add snap info or make a minor correction/adjustment to a certain part etc) you'll first need to unpack it using any zip format supporting tool. You might need to copy / rename it to .zip first though. In order to use the now unpacked/modified shadow library you also need to update the LDraw search location for the library using it. The dialog will let you choose ether a compressed file or normal folder to do this.
Editing the shadow version of .dat files can be done using any plain text editor. Before making any changes you might want to browse through some of the existing (bigger) .dat files and read the included ruleInfo.txt file to get started.
Information on how to define part snapping information for parts you'll find in the LDCad metas section.
When done working on a shadow library you might want to repack it again as processing the compressed version of it will be faster during loading of models etc.
Share your changes
If you add and or correct things in/to the official shadow library please consider mailing it to me. That way I can consider your changes / additions for future default shadow library releases benefiting all users.