The rules are whatever you want them to be. If the car spaces are common property, you pass a by-law that limits what can be stored on them.
If the car spaces are lot property, you pass a by-law that goods that are not in keeping with the appearance of the building may not be stored on car spaces.
Also, your local council’s planning regulations may say that car spaces in and around apartment blocks may only be used for parking cars.
My strata scheme by-laws says only a “roadworthy vehicle” can be left in the parking space, yet I have been instructed to move my pushbike (it’s no big deal, really). But it’s just occurred to me – there’s a jet ski on a trailer parked nearby. The trailer is roadworthy, but the jet ski considerably less so. Hmmm … wonder which “bush lawyer” on the committee I can stir up with this.