Equipment manufacturers often have clearance space requirements for their products. Reasons may include physical door swing or panel opening space, recommended access space, code required access space, thermal dissipation space, airflow space, etc.
For example, electrical distribution panels are required by code (N.E.C.) to have working clearance in front of them, extending some code-required width, height, and depth. Nothing is allowed to be in that space. Some people might like to build their families so that when they place them into a model, there is an indication of that space. This is for coordination purposes just to make sure they don't design something that's not code compliant or against manufacturers' reccommendations. There should be no harm in turning off the visibility of such items, and in fact you'll likely want to before you get everything all finished and ready for printing.
It would be nice if people stuck to the convention of using a subcategory for those clearance space items, so that we could turn them all off at once with a single change in the Object Styles manager. But not everyone does it that way, so check the other ways described.
