One instruction on the Mill can contain many operatkions that are issued together. Those operations can have data dependencies among each other. For that reason operations were divided into distinct catgories called phases that permit an ordering in consecutive cycles for those operations to be executed in to account for those data dependencies.
The Phases are:
1 (Reader) operations that load or create values into the belt with hardcoded arguments
2 (Operation) operations that take belt slots as arguments and produce results
3 (Call) function calls
4 (Pick) pick operation
5 (Writer) stores and branches