That's the point you'd make as part of your contract negotiation, prior to starting work, but it's not in the company's interests to volunteer that limitation. By doing so, they only create the headache for themselves of having to prove that something was within the scope of the restriction.
In some cases it would be obvious, like if someone invents a baby product whilst working for an aircraft company.
But, suppose you hire someone who, whilst working for you, comes up with an idea for something you don't currently make, but could. (Or maybe, a type of product which you already have plans to start making, or which you started making since the day the contract was signed).
Say your company makes batteries, and one of your researchers comes up with a novel charging method. You don't currently make battery chargers, but doing so would be a natural extension of your business, especially if you now have a unique selling point that your company could benefit from. The argument could go either way as to whether the invention was (or is, or could be, or could have been foreseen to be...) within the scope of your business.