http://processingjs.org/content/download/processing-js-1.2.1/processing-1.2.1.min.js
http://processingjs.org/content/download/processing-js-1.2.1/processing-1.2.1-api.min.js
Just observed that 2 javascript files are shipped in the distribution, what should I use for runtime functionality?
You could use either one, however the minified version is optimized for production because it saves bandwidth. If you plan on just using canned functionality from the script, the http://processingjs.org/content/download/processing-js-1.2.1/processing-1.2.1.min.js will do fine. However, if you wish to have the ability to change the code in the future, the api is what you want.