The coolkey applet is open source applet. Its source code [1] is available under dogtag project and is internally used by the Red Hat Certificate System.
The coolkey applet is closely bound to the PKCS#11 representation and is implemented in src/libopensc/{card,pkcs15}-coolkey.c and initially based on the Muscle applet.
The documentation is available online [2].
[1] https://github.com/dogtagpki/coolkey/
[2] https://github.com/dogtagpki/coolkey/blob/master/applet/doc/CoolKeyApplet.odt