In this case aliasing refers to memory aliasing. As in, given a common struct that represents a type union, you can overlap the fields that satisfy 'unmanaged'

In this case aliasing refers to memory aliasing. As in, given a common struct that represents a type union, you can overlap the fields that satisfy 'unmanaged' constraint (think int, MyStruct(short, long), Guid) between each other, but you cannot overlap them with fields that are or contain object references, because this violates the spec and GC will likely crash upon encountering data that turns into garbage when interpreted as a pointer.