The 'usual' distinction is how closely something is to the type of work your doing at the company. So, if your designing a CPU and you come up with a novel cashing algorithm it's owned by the company but if you come up with a novel shovel design it's independent. Which means there is still plenty of options to tinker in your own time but you can't sell an idea to a direct competitor.

The problem though is company's like IBM cover so many areas that things get vary fuzzy.