Calamus is Ornithe's intermediate mapping layer. The purpose of intermediate mappings is to provide a consistent base for named mappings. Each version of Minecraft is obfuscated slightly differently, which makes porting named mappings between versions tricky. Intermediate mappings are not meant to be readable names, making the porting process more straightforward. As intermediate mappings are the same across versions, and not constantly updated, like named mappings are, named mappings can more easily be ported between versions.
Calamus mappings are provided under the Creative Commons Zero license.