A *.lcs file type represents a single level of the hierarchy of a circuit. It is a zipped xml file that contains a single file named "root.xml" that represents the circuit. Here is the DTD specification of this xml file.
A *.lca file is a file type that is intended to represent the complete hierarchy of a circuit. Like a *.lcs file, it is a zip file that contains a file named "root.xml" in the root directory of the zip file. It may contain other files in the root directory that may be referenced by components at any level of the circuit hierarchy. There is a single subdirectory in a *.lca file named "subcircuits". This directory contains xml files with the extension *.xml that represent subcircuits referenced from "/root.xml" or other subcircuits. It also contains plain text files that are standard SPICE subcircuits that can be referenced from components at any level of the circuit hierarchy. The xml files in a *.lca file conform to the same DTD as the xml file in a *.lcs file.