Tech Tip: Booleans in Onshape
The Boolean command has three options: “Union,” “Subtract,” and “Intersect.” The “Union” option is useful if you want to merge multiple solid parts into one. Simply select all the parts you want to merge and they will be brought together to form one.The second boolean option, “Subtract,” allows you to subtract a part or parts from another one. This is a common technique in many different industries, such as mold making, where the first step in the process of creating a mold is subtracting a finished part from a cavity or core.
Source- Onshape Blog