LibraryLibrary is a re-useable composition of primitives. For making Lib, select the menu 'File'-'Export to Lib' after select primitives to save.
Selected primitives are saved to Lib as a ".sklib" file.
For use the Lib, select the menu 'File'-'Import from Lib' or just drag&drop to Skinman canvas from Explorer. the "sklib" file is loaded to current project.
Even if the Lib can be made by discretely selected primitives, I recommend to make 'Group' and name appropriately (both group-name and filename).