See the docs for the C API here: CCSMP API: Main Page (API Guide)
(Note, the JavaRTO API is a JNI wrapper around the C API… if you need pure Java, there is the new Java API, and the older JCSMP API)
It is a bitmask. So if you want core 4 (numbering starts at 0), you’ll want to set only the 5th bit from the right, or 0b00010000 , which is 16.
255 means 0b11111111 , or setting the mask so it can run on any core from 0-7.