Hi,
Well Sharada about the History tree there are 3 parts
1. The branch represents the feature i.e; whether it is a Prism(PRSM),Cube(CUB),Sphere(SPH) etc.
2. The node represents the operation like if it is a UNION(+),SUB(-), INTERSECTION(&),SPLIT(/) etc.
3. The terminal branch usually it is represented as *SOL1 represents the terminal or resulatnt solid i.e; the result of the booleans done amongst various solid to achieve the final output.
Well I hope this satisfies your query. If you have anything to discuss about let me know.
khan