Turbo Tax says of the special depreciation allowance (post September 17 2017) "you can elect to take a special allowance of 50% instead of 100%. Whichever you select will apply to all assets and vehicles that you acquired and started using during that timespan." I read that to say that if I choose to use a special depreciation allowance on one asset, I must use it on all. Certainly in the past, bonus depreciation could be applied to some assets, but need not be applied to all. I am now wondering that the statment means if you chose to, for example, use the 50% special allowance on one asset, you must use the same 50% allowance on any other asset that you choose to use the special allowance on. Not that you must use it on all assets.
It means all assets that use Special Depreciation Allowance.
However, your comment "in the past, bonus depreciation could be applied to some assets, but need not be applied to all" is not really correct. You can't randomly choose bonus depreciation for some assets and not for others. If you elected out of bonus depreciation, that applies to all assets in that "class". So if you elect out of a 7 year property, all 7 year property can't use bonus depreciation for that year.
https://www.irs.gov/publications/p946#en_US_2017_publink1000299556